Adding a link in the documentation, moving libsmet to the plugins subdirectory and small fixes in GRIBIO

The plugins used to keep a reference to a config object as provided by IOManager. But this was triggering some warnings (reference to an object that might disappear outside of the constructor), therefore it has been replaced by a const copy (it is not yet 100% sure if this is really better, but at least the compilers won't complain).

When a user was provinding IMISIO with a non-existing station name, the error messages were quite cryptic. In order to properly tell the user what happened, the code has been slightly reorganized. Some code clean up also took place in IMISIO.

Some better error messages have been added to SMETIO as well as some code cleanup. Some comments have been added to IOManager, and missing popc code added to DEMObject and Date.

The plugins are no longer separately loadable libraries, but are fully incorporated into libmeteoio. All classes and facilities that provided capabilites for the dynamic loading were deleted (DynamicLibrary). IOPlugin now holds a function pointer to an instantiation function for children of IOInterface. In IOHandler::registerPlugins() all plugins need to be registered with the key name, the class name, a pointer to an instance of the class and a function pointer to the instantiation function for the class (child of IOInterface). IOInterface lost some complexity and thus the constructor of the base class does not have to be called in derived classes anymore.

The CMakeLists.txt files had to be updated to no longer make separate dynamically loadable libraries for each plugin, but to simply add the classes the user selectes to the meteoio library. 

As a side effect the io.ini key GENERAL::PLUGINPATH is no longer necessary.

New warnings have been tried to detect variable shadowing (one variable declared with the same name as another one), alignement issues, etc. This produces very verbose output (not all warnings are legitimate) but allowed to fix lots of cases of misalignement and shadowing issues. These options have been left in the CMakeLists but NOT enabled (since these warnings are not so easy to interpret).

Some documentation fixes, new gradients (black to white, white to black and blue-white-red) defined and used for new variables in PNGIO (P, ALB and DW). A bug has been fixed for ILWR in GRIBIO as well as a bug when converting geographic coordinates to rotated ones.

New options have been added to the undercatch filter, allowing to specify the temperature thresholds with the CST argument (to make it more similar to GeoTop).

A new plugin appears: GRIBIO, to read GRIB files (see issue 19). This is still extremely preliminary alpha code (enough warnings?!), but it should (hopefully) quickly evolve. A small naming inconsistency has been fixed in template.cc and all plugins now keep a reference to a Config object instead of a full copy (which means that the only full copy of a Config object should now be in IOManager and all other components should have a reference to it). 

This addresses the 2D part of issue 168: the ability to specify which meteo parameter and time step to read/write from/to a 2D grid. This should be the prefered reading method in the future, since it leaves to the plugin the task of building the file name, extension, database table, etc This also provides added metadata and more control over the writing process.

A new method, getExtension() has been added to IOUtils in order to return the extension of a given filename. This is used by the SMETIO and SNIO plugins to automatically add the proper extension to input file names if they don't already have one.

The Median Average Deviation filter has been ported (and validated) to the new filtering infrastructure.

Several error messages have been improved.

GSNIO: Added support for RAIN_METER (HNW), SURFACE_TEMP (TSS) and SOLAR_RAD (ISWR) - possibly other values will need to be added.

ISSUE: GSN has up to two measurements per minute. We need to find a way to deal with this elegantly in the SMET output (currently only displaying minutes and not seconds for the timestamp), internally the julian date is used for comparisons.

SMETIO: Further development of the writeMeteoData procedure. Now all meteo parameters present in the MeteoData objects can be written (so not only the standard ones like MeteoData::TA, MeteoData::RH, etc). This lead to a slight reorganization of the code. Comments were added for private member functions.

GSNIO: More comments explaining the mapping between the GSN meteo data fields and the MeteoData meteo parameters.

The CMakeLists were not making Visual C++ happy. This has been fixed. Several types issues have been fixed. The /Wp64 option of Visual C++ has been turned off, since it reports wrongly some things as errors (dixit Microsoft and my own experience).

The OLWR hack for Norway has been implemented in SMETIO (a cleaner, long term solution will still have to be developed). Various types issues have been identified by Visual C++ and should have been fixed.

More size_t related issues have been found (thanks to the new snowpack server) and fixed. Basically, please remember that any variable used with/for the return value of a call to size() MUST be of type size_t (this becomes specially relevant in 64 bits).

The water triple point pressure has been updated to a better value.

A new library for dealing with SMET files has been developed. The plugin SMETIO builds on top of this library now. It provides:
- A SMETReader class, to read SMET files and parse the header info
- A SMETWriter class to write SMET files and set the header info

The SMET library (libsmet.h and libsmet.cc) are completely independent from the rest of MeteoIO. They have their own exception handling and helper functions, thus they can easily be distributed to other parties wishing to read and write SMET files with C/C++.

A new version of the fill method for arrays has been implemented: it only takes a sub-array and the lower left coordinate where to insert it. The file reading optimization that has been implemented in GeotTop and SNIO has also been implemented in SMETIO.

The TIME_ZONE keys are now used by ALL plugins (I thought I already did it...)

A new group of classes has been created, under the meteolaws subdirectory. This contains various general meteorological laws, such as a sun radiation model, standard atmosphere, etc

The first implementation (not tested yet) of a non-linear least square fit algorithm has been implemented in the libfit1D. 

Several documentation issues have been fixed (obsolete code examples, etc) and classes grouped by modules. This should help the user to quickly locate the classes of interest. 

The changes to the IOInterface (no more vec<vec<StationData>>) have been propagated into all plugins.

The SMETIO plugin now supports the "units_multiplier" field. The SMET specification has been brought to 1.0. A forgotten debugging output has been removed in InterpolationAlgorithms

The bufferAlways method of BufferedIOHandler has been renamed as setBufferProperties. The A3DIO, IMISIO, SMETIO and SNIO plugins now correctly support time zones. The plugin howto documentation now warns the developer to take care of time zones issues.

The DEMObject was not properly buffered (the update flag was not honored), this has been fixed. The A3DIO plugin now implements readStationData. A few speed improvements have been brought to A3DIO and SMETIO by using the "reserve" method of the vectors (so that the vectors don't have to reallocate memory constantly).

The WIND_CURV spatial interpolations algorithm now intelligently checks if the curvatures are available or not and gives a warning if not (and also take a 0 grade so that it would not be used).

The SMET plugin now uses the user provided coordinate system (as a default value for input and as a target coordinate system for output).

BUGFIX: When adding a key on the fly to the ConfigReader object it disappeared when accessing it in a plugin. Reason: The dynamic instantiation used the filename parameter in the constructor of the plugins instead of the ConfigReader object itself. Architectural change inside DynamicLibrary was necessary.

The EPSG get and set methods have been added to Coords. The "<<" operator has been redefined for ConfigReader (this is convenient for debugging). The SMETIO plugin has been updated (so that it supports EPSG codes and also to change some column names and improve the foutput formatting). Some documentation has been added and the example data set has been replaced by a data set suitable for the whole Switzerland and in SMET format.

The WSMDF format has been renamed into SMET. Therefore, the plugin has been renamed in SMETIO. Compilation and documentation have been updated accordingly.

The WSMDF plugin now sports a nicer formatting of columns in ASCII mode. TODO: unit_multiplier, unit_offset and GZIP

Added new plugin for WSMDF format. gzipped version of format currently not supported. binary version works, but since the julian date does not fit into a 4 byte (32bit) float, it is unusable. ASCII format works, although unit_multiplier and unit_offset are currently not supported. For location information the easting, northing version is not yet supported.

Added function IOUtils::stripComments(string&) it, deletes everything after # or ; from the string&