Refactor gmx_reverse_top_t

Refactor gmx_reverse_top_t

Made the impl properly private, but to do so had to separate
the responsibility for checking the local topology, which
should never have been here.

This introduces two new headers in preparation for splitting up
domdec_topology.cpp into several parts that can then have their own
headers. But first gmx_reverse_top_t has to be properly encapsulated.

Refs #3887