Memory-lean implementation of FOF

New strategy:

• Run FOF only on the linkables.
• Do all the union-finding business also over MPI.
• Assign the group IDs to all the linkables.
• (new) Run a new FOF loop over the particles this time on the attachable and find their nearest linkable. Record the distance for this and assign the linkable's groupID to the attachable.
• At this stage each particle knows which group it is in.
• Allocate arrays on each node to collect all the group info by looping over every particle.
• All-reduce these arrays.
• If needed, seed black holes.
• As all nodes now have all the info about every group, simplify the i/o and have only rank 0 dump the file.