Removed Selection::indexGroup().

Removed Selection::indexGroup().

This method provided access to the old C structure that was used in 4.5
to expose the selected atoms to help in converting code that used
certain old C functions.  Now this structure should be considered an
implementation detail of the selection module, and the new C++ API
should be preferred.  Changed remaining uses of the method to use
alternative approaches (which actually simplified select.cpp somewhat).

Change-Id: I8806b3e080b6fafd83d7634308f39f7335021376