Feature/subsamplingvtkwriter with refinement tag

The SubsamplingVTKWriter is extended to take advantage of the generalization of VirtualRefinement, explained in dune-geometry!51 (merged). Without any changes in the user code, the SubsamplingVTKWriter behaves as before, but a deprecation warning is issued. Instead use either

Dune::SubsamplingVTKWriter<GV> vtkwriter(gv,Dune::VirtualRefinementTag::Levels(subsampling));
Dune::SubsamplingVTKWriter<GV> vtkwriter(gv,Dune::VirtualRefinementTag::Intervals(subsampling));

Depends: dune-geometry!51 (merged).

Closes: dune-geometry!56 (merged).