Add FoF ouput to snapshots automatically

It would be extremely convenient to be able to have the stand-alone style friends of friends be run when writing each snapshot -- such that the FoF group IDs are added to the snapshot for each particle -- in simulations that do not need the on-the-fly FoF to be running (e.g. planetary).

The current alternative is to manually run the stand-alone FoF after the snapshots have been produced.