Clang format run on near-the-header lines.

Happy New Year

This has some follow-up changes.

Don't repeat typename in emplace_back.

MSS---the mesh subsets.

MSS are the MeshSubsets.

This is just updating the interfaces and some of the for-loops
but will not compile until the using part is also updated.

MCMap now also accepts nullptr MeshSubsets*.

Remove template specialization and name the functions
correspondingly.
Improve documentation.