This VTKWriter is then used for writing the individual time steps.

Advantages:
- Allows to write sequences of dune-functions grid functions.  Previously, this was
  not possible, because the corresponding forwarding addCellData/addVertexData methods
  of VTKSequenceWriter are not implemented.
- It makes the SubsamplingVTKSequenceWriter class obsolete, because you can now simply
  construct a VTKSequenceWriter with a SubsamplingVTKWriter as its writer object.
- It makes the addCellData/addVertexData methods of VTKSequenceWriter obsolete,
  as one may now call the corresponding methods on VTKWriter directly.

(cherry picked from commit 519c3482

)
Signed-off-by: Steffen Müthing <muething@dune-project.org>