Apologies in advance for the monster push...
This adds support for black hole particles. We can:
- Read BHs from ICs,
- Write BHs to snapshots,
- Exchange Bhs over the network,
- Put the BHs in the correct cells,
- Integrate them forward in time.
This should not break any of the simulations we do until now and the EAGLE-low-z examples should run with the new runtime flag --black-holes. This ought to work with and without MPI.
Actual tasks using the BHs will follow but having this in place first is helpful.