Fix logical problems in MGConstrainedDoFs and rewrite step 16

MGConstrainedDoFs had a constructor that could result in an inconsistent state. Furthermore, it assumed Dirichlet boundary only. Timo and I reorganized the class, removed part of its data, and made sure that it does what it promises.

In this process, in order to remove deprecated functions in the future, and also in preparation for parallel multigrid, step 16 was changed to use MeshWorker.

Some tests needed minor changes, but their outputs have been verified.
That's awkward since doxygen will not be able to cross-link the members from these two namespaces properly in the online documentation. (It gets namespace dealii right because we pretend to doxygen that there is no namespace dealii at all.) Would it be possible to explicitly qualify everything from these latter two namespaces?