Revision 1964

* Thuban/Model/resource.py: Rework the way gdal support is
determined so that we can give a reason in the about why gdal is
not supported.
(gdal_support_status): New. Variable holding a string with the
reason for no gdal support

* Thuban/UI/about.py (About.__init__): Add the reason why gdal is
not supported to the message