Additional index group checks for selections

Additional index group checks for selections

Now index groups provided to selections are checked more thoroughly, and
an error is given if a group references an atom index outside the
topology (or the maximum atom number in the absence of a topology).
Negative atom indices also trigger the same error.

Previously, such index groups were silently accepted (except for a
warning during reading for negative indices), and the out-of-range atoms
dropped from the selections.

Change-Id: Id7a024ed316920d9594454ae36f7b9bdd9a429fc