Try Meteosat diffuse radiation for sclaing of shortwave radiation

[jlandmann]

To scale shortwave radiation in terrain with the helpt of potential radiation, we currently use Meteosat total shortwave incoming radiation, assuming a fixed proportion of 20% diffuse radiation. This ensure that radiation does not become zero, but is a very poor solution. We should go ehead and try to use the Meteosat diffuse radiation for the downscaling as well.