Cleverer ids for particle splitting
Two updates:
- Add the option to remap all the particle IDs to [1, N] after having read the ICs,
- Add the option (set as default) to use the next available IDs when splitting particles rather than random ones that can potentially create conflicts.
This is useful for the set of EAGLE runs we want to do.
@pdraper any thoughts on the implementation are welcome.
If/when we are all happy, I'll modify all the relevant parameter files and update the documentation.