By simplifying MeteoData, the memory consumption of a benchmark such as data_converter is reduced by almost 20%

Added a better support of Clang on osX in Cmake; fixed lots of the warnings generated by the "Weveryting" option. Still some work left to do, though\!

Now multiple DEMS can be cached. The implemented solution is not fully satisfactory (a API change for all plugins would be required), but at least the core of it is implemented (in the Buffer class).

The GridBuffer class has been expanded and is now used by Meteo2Dinterpolator for interpolated grids

A gridBuffer class has finally been implemented (basically, moving the old grid buffering code into it) and is now in use for the GridsManager. An invalid file name in the examples has been fixed and a toString() debug message has been improved in the TimeSeriesManager.

the GSN plugin now has a "GSN_DEBUG" key to print out the full dialog with the server in case of errors. Some headers returned by GSN did not follow what we expected (multi-lines description) and this is now supported. New sensors names are also supported so the Stillberg data can be read.

The buffers have to extra setters to manually set the start/end dates (this is quite a hack but will be required for the first implementations relying on them).

Slightly improved toString() output as well as first version of a proper Buffer class (not used yet)