User-creatable index lists
Users should be able to create index list classes if they can describe their indices more succinct this way. The following parts would be needed for such:
- Registering a pack tag and making sure it's identical on all ranks.
- Registering an unpack function.
- Optionally registering intersection function(s) for different (other) index lists and extending the table in xt_idxlist_intersection.c correspondingly.
Anything I forgot?