Use gmx_mtop_t in selections, part 2

Use gmx_mtop_t throughout low-level selection routines, i.e.,
centerofmass.cpp, poscalc.cpp, and indexutil.cpp.  Adapt test code,
which is now using gmx_mtop_t throughout as well.

In places where gmx_mtop_t is actually accessed, the changes are as
local as possible.  In most cases, some additional restructuring could
give better performance and/or much clearer code, but that is outside
the scope of this change.

Part of #1862.

Change-Id: Icc99432bddec04a325aef733df56571d709130fb