Allow FOF to run on a subset of the particle types

For @zbelcheva's project, we will need to run FOF on the stars only. At the moment, the code cannot handle that and uses all the gpart irrespective of their type.

We should introduce an option to run only on a specific subset of swift_part_types. For instance, all stars, or stars + BH. Making it generic is better than just having an ad-hoc solution for that specific project.

It should be not too hard to do. All we want is select the gpart. We don't actually need their part/spart/bpart friends.