Stephen's Ideas on hop for Rick to implement

- It would be super useful if a record of where star particles were when they were created was kept. In particular for my current needs, it would be nice to say definitively if a star particle was in or outside a halo when it was created.

- Make some record of each group a particle has belonged to over the course of the simulation.

- It would be nice if the group IDs were constant, as in the first identified group was group 0 for all time, the second as 1, etc... This would make tracking particle membership more straightforward. Since groups move and change membership, it would be up to a simple nearest group search to link groups from one time step to another.