Add unit tests for index groups in selections.

Add unit tests for index groups in selections.

Add tests for using the "group" keyword in selections to select external
index groups, and in particular for directly providing an index group as
the selection value.

Fixed a master-only issue in handling the selection name in the latter
cases.  Also fixed master-only memory leaks and other issues in
gmx_ana_indexgrps_init().

Part of #651.

Change-Id: I0d908213f109ced9ab6708e55c6acf53371f6aa1