Small clean up for the specific/relative humidity conversions. Small code clean up (mostly indentation) for NetCDF.

NetCDFIO: Now all the wrappers for libnetcdf have been put into a separate namespace called ncpp in separate files.

NetCDFIO: Full implementation of the STRICTFORMAT config option. By default all parameters, also those not conforming with the format are read and written, the user though may force conformity by setting the STRICTFORMAT parameter to TRUE in the io.ini [Input] and [Output] sections

NetCDFIO: Introducing the STRICTFORMAT config parameter for the io.ini when using the NetCDFIO plugin to read or write meteo data. This paramater is by default set to false meaning that meteo parameters that are not present in the CNRM standard for NetCDF files will also be read and written. Setting STRICTFROMAT to true means that only parameters defined in the CNRM standard will be regarded when reading and writing meteo data, all other meteo parameters will be ignored.

A few extra checks have been added for grid rescaling. Some indentation has been fixed and lots of variables made const. The documentation is now linked to the general plugins documentation.

NetCDFIO: Further cleanup, readStationData revamped, added a method to check dimensions of a variable

NetCDFIO: Thanks to heavenly input by Mathias the 2D grid reading and writing is numerically consistent now. The only tradeoff is that precision of the cellsize is limited to 1cm, otherwise numerical instabilities may occur. 

NetCDFIO: deleting existing files before creating a new one when writing meteo data, everything else is too complicated for the moment

NetCDFIO: reading of meteo data from FORCING.nc implemented, cleanup and correct conversion of units necessary.

The cmake files for compiling with LibXML++ have been updated since they would not work with more recent Linux (but we should consider dropping LibXML++ and only rely on LibXML2 in order to reduce the dependencies and package finding mess). The special points are now called POI (Points of Interest, like for a gps device) in order to make them more familiar.

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.

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.

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.

Fixed some bad naming in DynamicLibrary, silenced an unnecessary warning from Visual C++ (urging us to use non standard library calls), fixed some DLL exports for Visual C++.

The arrays are now using the vector filling methods when possible. A specialized template for convertString(unsigned int) has been written. 

A two segments linear fit (for elevation detrending) has been implemented with a fixed inflection point. This is quite primitive but allows the LLIDW spatial interpolation to use it and therefore to behave like the traditional SLF SnowFit program. More work will be needed for fully configurable regression algorithm choice though... 

Some namming issues have been fixed in COSMOXML. A better emissivity handling has been implemented in SNIO to be closer to the original Snowpack (ie: compute clear sky ILWR when no cloudiness/emissivity is given).

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