[dune-pdelab] Implementing OVLP_AMG_4_DG solver

[Christian Engwer]

Hi Lukas,

> Grid: YaspGrid
> DG Space: OverlappingEntitySet, QkDGLocalFiniteElementMap (order >0), NoConstraints or P0ParallelConstraints
> CG Space: OverlappingEntitySet, QkDGLocalFiniteElementMap (order 0),P0ParallelConstraints

Please try OverlappingEntitySet, P0LocalFiniteElementMap (order 0), P0ParallelConstraints

I'm unsure what the Qk does in the case order 0, but I'm afraid it
might to stupid things...

Christian