Calculate birth halo catalogue index

[robjmcgibbon]

• Added a script which loops over all the membership files from a run to find out what subhalo each star was bound to when it first appeared. It is also capable of calculating which subhalo the gas progenitor was bound to.
• Updated the virtual snapshot script so it can take multiple "auxiliary snapshots" and combine them to a single virtual file. This script allows us to create a snapshot with all of the original properties, the membership information, and the birth information
• Added an ExSitu property to SOAP. This requires the newly generated files to be passed as ExtraInput within the parameter file.

One thing to note is that for gas progenitors I don't handle particle splitting properly. I simply look for a gas particle in the previous snapshot with the same ID as the newly formed star. This fails if a particle has both split and then formed a star between consecutive snapshots. If we want to update this script to handle it properly then we should make use of https://github.com/SWIFTSIM/HBT-HERONS/tree/master/toolbox/swiftsim/particle_splitting.

TODO:

• Write some short documentation on the format of the output