Make LocalTopologyChecker class and hide implementation details

Make LocalTopologyChecker class and hide implementation details

This allows capturing useful state that is only used when writing the
error output, without leaking dependencies for implementation details
to high-level code.

Some minor improvements to const correctness of the error output. Also
moved it into gmx namespace.

Minor improvement to include order

Removed unused forward declaration in domdec.h

Refs #3887