snowfall and rainfall outputs

[mee067]

Some code versions used to output snowfall and rainfall, as separated by CLASS depending on the IPCP flag value (CLASS). This was disabled at some point and it affected r1860. It was still not fixed for 1.5.1.

To resolve this, lines 474, 475 in `model_variables.f90' need to be uncommented.

          allocate(group%prern(n), stat = z); if (z /= 0) ierr = z
          allocate(group%presno(n), stat = z); if (z /= 0) ierr = z

Dan mentioned he did comment them intentionally to solve some issue with SVS. Hope that issue is being resolved/isolated without affecting CLASS.