Fixing problems with MGConstrainedDoFs
implement deprecated get_refinement_edge_boundary_indices()
use new logic in multigrid assemblers and switch step 16 to MeshWorker
all existing multigrid tests verified with fixed MGConstrainedDoFs
new test for Neumann data
Update documentation of step 16
Fix minor issues in multigrid tests
Merge pull request #43 from guidokanschat/new_multigrid_tests

Fix logical problems in MGConstrainedDoFs and rewrite step 16