Remove overeager selection index group check

The check for sorted groups in selections was a bit too eager, and
prevented use of many index groups that would previously have been
perfectly valid for, e.g., g_bond, with gmx distance.  Remove the
check.  Instead, fix the case where unsorted group is passed as the
value of a selection, and add tests for that.

Will reinstantiate the check later in another form, as currently
unsorted index groups get silently accepted and treated in a more or
less random manner in all other cases.  However, this change provides
a usability improvement that could be nice to have, even if I don't
immediately have time to implement the check.

Change-Id: I86e935d8bd08e63a9e3d01fb5a4084d96c5b7006