Move gmx_ana_pos_t::g into gmx_ana_indexmap_t.

Move gmx_ana_pos_t::g into gmx_ana_indexmap_t.

Instead of storing a gmx_ana_index_t structure by reference in the
positions, embed this information in the index mapping structure, as
that is really what it is required for.  This localizes the
responsibility of maintaining that state better, and allows removing
code that was there just to provide such an artificial group structure.
The logic still remains mostly the same: the array of atoms is still
stored by reference instead of copying where possible.

This also makes it simpler to handle cases where the output atoms from
the mapping would actually not equal the input group (e.g., that they
would need to be in a different order).

Prerequisite for #1221.

Change-Id: I9e3e0455d1129fd6c3dd8056b2f088114764f331