Basic infrastructure for black holes

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.

doc/RTD/source/CommandLineOptions/index.rst

@@ -30,6 +30,7 @@ can be found by typing ``./swift -h``::
     -M, --multipole-reconstruction    Reconstruct the multipoles every time-step.
     -s, --hydro                       Run with hydrodynamics.
     -S, --stars                       Run with stars.
+    -B, --black-holes                 Run with black holes.
     -x, --velociraptor                Run with structure finding.
     --limiter                         Run with time-step limiter.