Adding a new meteo parameter: the cloud transmissivity TAU_CLD defined as iswr_meas / iswr_clear_sky (ie the potential radiation computed at ground level with a standard atmosphere, see Iqbal 1983). This parameter is already used by the AllSky_LW and PotRad generators and properly handled by the PNG plugin. The PNG plugin can now also properly output a hillshade (as MeteoGrids::SHADE).