+Add unit conversion capability for restarts

  Added optional arguments to framework code to permit the restart files to work
with dimensionally unscaled variables.  All answers are bitwise identical, but
there are new optional arguments for multiple public interfaces.

 - Added a new conversion factor element, conv, to the field_restart type to
   specify how a field is to be unscaled before writing, and whose reciprocal is
   used to scale restart fields as they are read in.

 - Added the new optional argument conversion to each of the
   register_restart_field() and register_restart_pair() routines, which is used
   to specify how each field will be rescaled when it is written to or read from
   a restart file.  The default is the same as setting this to 1.

 - Added the new optional argument unscaled to fix_restart_scaling() and
   fix_restart_unit_scaling() to reset the unit conversion factors that will be
   save to the restart files to 1, consistent with writing out fields without
   scaling to the restart files.

src/core/MOM_verticalGrid.F90

@@ -188,10 +188,17 @@ subroutine verticalGridInit( param_file, GV, US )
 end subroutine verticalGridInit
 
 !> Set the scaling factors for restart files to the scaling factors for this run.
-subroutine fix_restart_scaling(GV)
+subroutine fix_restart_scaling(GV, unscaled)
   type(verticalGrid_type), intent(inout) :: GV   !< The ocean's vertical grid structure
+  logical,       optional, intent(in)    :: unscaled !< If true, set the restart factors as though the
+                                             !! model would be unscaled, which is appropriate if the
+                                             !! scaling is undone when writing a restart file.
 
   GV%m_to_H_restart = GV%m_to_H
+  if (present(unscaled)) then ; if (unscaled) then
+    GV%m_to_H_restart = 1.0
+  endif ; endif
+
 end subroutine fix_restart_scaling
 
 !> Returns the model's thickness units, usually m or kg/m^2.