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.

src/core_init_atmosphere/mpas_init_atm_cases.F

@@ -4004,7 +4004,7 @@ subroutine init_atm_case_gfs(block, mesh, nCells, nEdges, nVertLevels, fg, state
             sorted_arr(2,k) = rh_fg(k,iCell)
          end do
          call mpas_quicksort(nfglevels_actual, sorted_arr)
-         do k=1,nVertLevels
+         do k=nVertLevels,1,-1
             target_z = 0.5 * (zgrid(k,iCell) + zgrid(k+1,iCell))
 !           scalars(index_qv,k,iCell) = vertical_interp(target_z, nfglevels_actual, sorted_arr, order=1, extrap=0)
             scalars(index_qv,k,iCell) = vertical_interp(target_z, nfglevels_actual-1, &