Make NonoverlappingOperator work with BCRSMatrix of pods.

This should make support for scalar matrices more complete. Sequentially these matrices work, also for the overlapping case. It is just the nonoverlapping that was missed.

@gruenich Your choice whether this is release material. Either PdeLab does not use scalar matrices yet, or nobody tested the non-overlapping stuff with it.

@oliver.sander Can you check, whether I used the Hybrid stuff correctly? I just looked at your changes to gsetc.hh as an example for this.