Improve MOM_surface_chksum

  Added checksum calls for the melt_potential, ocean_mass, ocean_heat and
ocean_salt elements of the surface state in MOM_surface_chksum if these fields
are allocated for more comprehensive debugging.  Also added the symmetric
optional argument to the call to MOM_surface_chksum form extract_surface_state
so that all of the surface velocity values that could contribute to the ocean
surface velocities that are seen by the coupler are checksummed.  All solutions
are bitwise identical, but there are enhancements to the MOM6 debugging
capabilities.

src/core/MOM.F90

@@ -4028,7 +4028,7 @@ subroutine extract_surface_state(CS, sfc_state_in)
     endif
   endif
 
-  if (CS%debug) call MOM_surface_chksum("Post extract_sfc", sfc_state, G, US, haloshift=0)
+  if (CS%debug) call MOM_surface_chksum("Post extract_sfc", sfc_state, G, US, haloshift=0, symmetric=.true.)
 
   ! Rotate sfc_state back onto the input grid, sfc_state_in
   if (CS%rotate_index) then

src/core/MOM_checksum_packages.F90

@@ -167,7 +167,7 @@ subroutine MOM_surface_chksum(mesg, sfc_state, G, US, haloshift, symmetric)
   logical :: sym
 
   sym = .false. ; if (present(symmetric)) sym = symmetric
-  hs = 1 ; if (present(haloshift)) hs = haloshift
+  hs = 0 ; if (present(haloshift)) hs = haloshift
 
   if (allocated(sfc_state%SST)) call hchksum(sfc_state%SST, mesg//" SST", G%HI, haloshift=hs, &
                                              scale=US%C_to_degC)
@@ -182,6 +182,14 @@ subroutine MOM_surface_chksum(mesg, sfc_state, G, US, haloshift, symmetric)
                   scale=US%L_T_to_m_s)
   if (allocated(sfc_state%frazil)) call hchksum(sfc_state%frazil, mesg//" frazil", G%HI, &
                                                 haloshift=hs, scale=US%Q_to_J_kg*US%RZ_to_kg_m2)
+  if (allocated(sfc_state%melt_potential)) call hchksum(sfc_state%melt_potential, mesg//" melt_potential", &
+                      G%HI, haloshift=hs, scale=US%Q_to_J_kg*US%RZ_to_kg_m2)
+  if (allocated(sfc_state%ocean_mass)) call hchksum(sfc_state%ocean_mass, mesg//" ocean_mass", &
+                      G%HI, haloshift=hs, scale=US%RZ_to_kg_m2)
+  if (allocated(sfc_state%ocean_heat)) call hchksum(sfc_state%ocean_heat, mesg//" ocean_heat", &
+                      G%HI, haloshift=hs, scale=US%C_to_degC*US%RZ_to_kg_m2)
+  if (allocated(sfc_state%ocean_salt)) call hchksum(sfc_state%ocean_salt, mesg//" ocean_salt", &
+                      G%HI, haloshift=hs, scale=US%S_to_ppt*US%RZ_to_kg_m2)
 
 end subroutine MOM_surface_chksum