[!348] Make NonoverlappingOperator work with BCRSMatrix of pods.

Merge branch 'feature/novlap-scalar' into 'master'

ref:core/dune-istl 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.

See merge request core/dune-istl!348

(cherry picked from commit 88382dca)

c620c02c Make NonoverlappingOperator work with BCRSMatrix of pods.