do file back up in `ff_map`

[csbrasnett]

use vermouth deferred file writer like we do in ff_inter already

[csbrasnett]

Not currently possible with the vermouth file writer to interface with MDAnalysis writing. Need to do something like:

        outpath = pathlib.Path(args.outpath)
        with DeferredFileWriter().open(filename=str(outpath), mode='w') as xtc_out:
            with mda.Writer(xtc_out.path,
                            multiframe=True,
                            n_atoms=len(cg_universe.atoms)) as mapped:
                for time_step in cg_universe.trajectory:
                    mapped.write(cg_beads)

in ff_map, but this the DeferredFileWriter has no .path attribute. Maybe can be fixed if we can be bothered to put it in.