Add Selection::hasOnlyAtoms()

Add a convenience function for checking whether a selection consists of
atom positions only.  This will be useful for analysis tools that may
only support atom positions with some combination of options, leads
to clearer code than checking type() == INDEX_ATOM, and exposes less
internal implementation details.

Change-Id: Iaa2d0bfa2f567c060f6b006d423c2910ffd46a69