Defining additional dimension in restart files

[c2xu]

While working on PR #772, I encountered the issue of not able to create additional dimensions in the restart file. It seems to me the issue is related to the save_restart subroutine in MOM_restart.F90. Specifically, the extra_axes to be created in the restart file is generated in lines 1427 to 1429:

      call query_vardesc(CS%restart_field(m)%vars, hor_grid=hor_grid, &
                         z_grid=z_grid, t_grid=t_grid, caller="save_restart", &
                         extra_axes=extra_axes)

However, this call is placed within a loop, such that extra_axes generated for a restart field will be overwritten by the next restart field. As a result, when creating the restarting file in lines 1505 to 1511:

    if (CS%parallel_restartfiles) then 
      call create_MOM_file(IO_handle, trim(restartpath), vars, next_var-start_var, &
          fields, MULTIPLE, G=G, GV=GV, checksums=check_val, extra_axes=extra_axes)
    else 
      call create_MOM_file(IO_handle, trim(restartpath), vars, next_var-start_var, &
          fields, SINGLE_FILE, G=G, GV=GV, checksums=check_val, extra_axes=extra_axes)
    endif

the extra_axes is always the one associated with the last restart field.

In PR #722, I was able to get around of this issue by registering the restart fields of the MOM_streaming_filter module after all other restart fields/pairs are registered, but I think there should be a better solution. I've looked around and see that the MOM_internal_tides module also defines additional axes in the restart file, and I think there could be a problem if both modules are being used in the simulation.