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.