Flexible choice of VW source for snow drift calculations

A key WIND_DRIFT_SOURCE could be implemented to allow to choose which wind speed is used for drifting snow calculations. For example: WIND_DRIFT_SOURCE = VW / VW_MAX / VW_DRIFT

Has low priority, since MeteoIO editing's syntax can be used to copy parameters and set an appropriate VW_DRIFT (the current variable used for drifting snow calculations).