Merge branch 'bugfix_t2tv' into hotfix-v7.1 (PR #829)

When calculating moist air density in the real-data initialization case,
virtual temperature, rather than temperature, should be used:

Tv = T * ((1 - qv) + Rv/Rd * qv)
   = T * (1 + (Rv/Rd - 1) * qv)

Author: mgduda

src/core_init_atmosphere/mpas_init_atm_cases.F

@@ -4773,7 +4773,8 @@ subroutine init_atm_case_gfs(block, mesh, nCells, nEdges, nVertLevels, fg, state
             t(k,iCell) = t(k,iCell) * (p0 / pressure(k,iCell)) ** (rgas / cp)
 
             ! RHO_ZZ
-            rho_zz(k,iCell) = pressure(k,iCell) / rgas / (p(k,iCell) * t(k,iCell))
+            rho_zz(k,iCell) = pressure(k,iCell) / rgas / (p(k,iCell) * t(k,iCell) &
+                              * (1.0 + (rvord - 1.0) * scalars(index_qv,k,iCell)))
             rho_zz(k,iCell) = rho_zz(k,iCell) / (1.0 + scalars(index_qv,k,iCell))
          end do
       end do