Specify conversion factors in write_energy

  Revised write_energy to use conversion arguments to var_desc to unscale
variables.  Their units are also documented in the same calls, so this is now
analogous to what is done the register_diag_field calls for diagnostics that
are handled by the MOM_diag_mediator.  All calculations in write_energy and
almost all internal variables are now in rescaled units.  All answers and output
are bitwise identical.