Convert gmx_mtop_t to C++

Convert gmx_mtop_t to C++

gmx_mtop_t now uses std vectors for moltype and molblock.
gmx_mtop_t, gmx_moltype_t and gmx_molblock_t now have constructors
and destructors.
Changed declarations of gmx_mtop_t* to not use a pointer or to use
std::unique_ptr.
Several sub-structs still use C style pointers and will be converted
later.

Change-Id: Iee802e4382a0a389496bb9395389e5926918d0f8