Support for sink particles

[robjmcgibbon]

Jon is in the process of adding sink particles to swift. We need to consider how to support them in HBT.

• Sink particles can spawn stars. I think we can just update the particle splitting script to consider this information.
• We need to decide whether to reattach sink particles, but that probably isn't needed.

[VictorForouhar]

Confirmed that sink particles do not need reattachment, since they are collisionless and hence not gas-like.