YAXT

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?