bug-fix: convert salinity to MSU in MOM6 model_interpolate

fixes #509
The WOD observation converter is in MSU.

obs_def_ocean_mod.f90 does not have any non-COMMON_CODE forward
operators that depend on salinty. But this unit conversion is something
to keep in mind for future forward operators.

Author: hkershaw-brown

models/MOM6/model_mod.f90

@@ -49,7 +49,7 @@ module model_mod
 
 use obs_kind_mod, only : get_index_for_quantity, QTY_U_CURRENT_COMPONENT, &
                          QTY_V_CURRENT_COMPONENT, QTY_LAYER_THICKNESS, &
-                         QTY_DRY_LAND
+                         QTY_DRY_LAND, QTY_SALINITY
 
 use ensemble_manager_mod, only : ensemble_type
 
@@ -383,6 +383,11 @@ subroutine model_interpolate(state_handle, ens_size, location, qty, expected_obs
 ! expected_obs = bot_val + lev_fract * (top_val - bot_val)
 expected_obs = expected(:,1) + lev_fract(:) * (expected(:,2) - expected(:,1))
 
+if (qty == QTY_SALINITY) then  ! convert from PSU (model) to MSU (obersvation)
+   expected_obs = expected_obs/1000.0_r8
+endif
+
+
 end subroutine model_interpolate