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.

More refinements to the Winstral spatial interpolation: it is now possible to provide a reference station to take the wind direction from, or to provide a fixed wind direction. 

Fixed a stupid bug in my previous commit and added the possibility to provide a fallback constant when using the CST spatial interpolation:
RH::algorithms = Cst
RH::cst                = 0.3

This would behave as before, except that if no data at all would be available at a given time step, then the value 0.3 would be taken to fill the grid.

Since IOUtils was getting bigger and bigger, all the methods dealing with low level file handling have been moved into a new file, FilesUtils. It remains in the IOUtils namespace. One method has been moved to Date. A few includes have been cleaned up.

The example io.ini file was missing some new changes. The Winstral algorithm had a bug when operating on dems with holes (nodata). It now takes its wind direction as argument (the base algorithm also has to be provided).

some unecessary lines have been removed and a new target has been added: cxxflags. This only takes compiling options from the cxxflags env variable and is therefore suitable for packagers who need to force their own compiling flags.

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

A new constructor has been implemented for the GridObjects as well as better members alignement. A potential issue with Winstral has been fixed. A wind direction algorithm has been implemented according to Ryan, 1977.

Several bugs have been fixed: in the AllSky generator, the cloudiness was still forced to 0.5 (as for debug). The data creators were also called *after* the generators, thus making it impossible to used a created data in a generator. 

The placeholders have been put in place (but commented out) for the cloudiness/tau_cloud/clearness index support in MeteoData. It seems that Kt=clearness index might be the proper way to go, but this is still a messy field. Some comments have been added in the code to clarify the use of such parameters.

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

Data Creators have been defined: it is now possible to create an extra parameter from a data generator. Simply define it as {param_name}::create = {generators} in the [Generators] section and the parameter will be created from any data generators specified (beside the "create" syntax, everything is currently identical with data generators).