Add support for `SubhaloOccupation` class

[aphearin]

The feature being requested here is to be able to transfer the subhalo population from a high-resolution simulation into a larger-volume, lower-resolution simulation. There should be support for masked selection of the subhalos being transferred from one simulation to the other, so that the selection function can be parameterized and marginalized over.

@duncandc should weigh in here in case there are variations on this that would also be useful.