Adds the following:
- self/pair tasks for the black holes
- MPI communication tasks for the black holes
- A new black holes properties structure to contain the BH model details
- New type of black holes for the EAGLE model
- Bondi + Eddington accretion model for the EAGLE AGN model
- Update to the yaml parameter files for the new parameters
- Update to the python analysis scripts
- Update to the low-z ICs to contain black hole smoothing lengths.