Reinstantiate unsorted index group check

In most contexts, providing an index group with duplicate atoms or atom
indices not in ascending order results in the selection getting silently
evaluated incorrectly.  The selection engine now detects this case and
gives a fatal error, hopefully reducing confusion if such a case arises.

The case where an unsorted index group is provided as a sole value of a
selection is still supported.

Change-Id: I0b647a26d9a1b47da10c84bdd3d073412d166a75