Also apply the distance criterion when checking for BH mergers. Read the correct value of G from the physical constant structure.

Import small fixes from the blackhole swallowing branch that are unrelated to the swallowing itself.

Only call engine_rebuild() and not engine_init_particles() in main_fof. We don't need the full thing.

Better --fof help string and prevent the code from running with black holes seeding if the black holes policy has not been chosen.

Added communication functions to propagate the new number of star particles after a SF event has occured.

Prevent users from running with cooling switched on and no minimal temperature in the parameter file.

Only initialise the potential after the space has been initialised as the potential requires the size of the space.

Added basic support for black hole particles. Particles can be read in, attached to gpart, integrated forward in time, drifted and written to snapshots.