This function predates BlockSparsityPatternBase<T>::add_entries(), which does
the same thing (essentially just updating the individual blocks after computing
offsets). For good measure I added a test which verifies that we get identical
output with constraints and a DoF mask table before and after the switch.

We can avoid some expensive parts and looping over data more than once by
utilizing the fact that the DoFs are sorted, which implies that they are also
sorted by block.