Black hole swallowing

To do:

1. Swallow gas particles non-MPI. (done as of 2a1a00ef)
2. Swallow gas particles over MPI. (done as of a3d031d3)
3. Update the gas neighbour mass for feedback. (done as of 64e282fd)
4. Save metallicities and other gas properties when swallowing (done as of 321ac029)
5. Handle the case of SF particle being swallowed
6. Handle the case of stellar feedback particle being swallowed
7. Speed up the do_swallow() task by recursing where needed. (done as of 0efb0f11)
8. Update all the hydro schemes. (done as of 99a14b3c).
9. Swallow BH particles.