MTNG compatibility

[CianMRoche]

Some functions not compatible with MTNG (MillenniumTNG) data types and offset structure, in particular:

1. il.groupcat.loadSingle() with the subhaloID set was having problems with the integer datatype "<u6". Line 219 I believe is the problem, giving the error: TypeError: data type '<u6' not understood. Cast to numpy-friendly int necessary when appropriate seems like the fix

2. The field "Group/SnapByType" does not exist in the offset files for MTNG, leading to the error KeyError: "Unable to synchronously open object (object 'SnapByType' doesn't exist)" when calling il.snapshot.loadHalo for example. The "Group/SnapByType" and "Subhalo/SnapByType" fields in the offset files for TNG (<simulation_dir>/postprocessing/offsets/offsets_xxx.hdf5, shape (large N,6)) indeed seem to be replaced by the "Group/GroupOffsetType" or "Subhalo/SubhaloOffsetType" fields in the MTNG group catalogue (<simulation_dir>/output/groups_xxx/fof_subhalo_tab_xxx.n.hdf5, shape (small N,6)). Notably these files are broken into chunks whereas I think the TNG version is not.