Updating version number to 3.1
Fix bug in extrapolation of RH below the surface for MPAS-Atmosphere

The change to a constant extrapolation of RH below the surface in
commit 21d4043 crucially depends on levels being processed from top
to bottom, so that a given level can refer to the level above.

This commit corrects the vertical level loop used in the vertical
interpolation of RH, so that levels are processed top to bottom.
Merge branch 'hotfix_v3.1'

This merge corrects a bug in the extrapolation of RH
below the surface for MPAS-Atmosphere.