Towards enabling netcdf compression in restart file

• Use ensure_netcdf_var_exists in gs2_save::gs2_save_for_restart

  This reduces boilerplate code and will enable us to easily enable compression etc. with a change in just one place.

• Use ensure_netcdf_dim_exists

So far this just replaces direct calls to nf90_def_var and nf90_def_dim with the thin wrappers in netcdf_utils. This lays the groundwork for enabling compression through changes in netcdf_utils (note we have yet to update the utils commit here so we do not yet have access to this functionality).

Adds USE_NETCDF_COMPRESSION make flag. If defined then we default to using compression in the restart files (and other files which use the netcdf utils wrappers).