Allow Q(p) and DGQ(r) elements to touch each other in hp::DoFHandlers.

The thing is that the FE_DGQ is discontinuous anyway, so there can't be any constraints to begin with.
Merge pull request #175 from bangerth/fix-q-vs-dgq

Allow Q(p) and DGQ(r) elements to touch each other in hp::DoFHandlers.