Rename `snap_molecule_cluster` since the input isn’t always a snap

[jennyfothergill]

@gwenwhite mentioned some confusion with this error from snap_molecule_cluster and _validate_inputs.

ValueError: Only pass in one of snapshot, gsd_file

One way we can help to make this more clear is to rename snap_molcule_cluster so it doesn't have "snap" in the name. As it operates on the bonds, maybe something like get_bonded_cluster?