Merge branch 'feature/improve-vtk-sequence-writer' into 'master'

ref:core/dune-grid

-   [x] Expose clear() function of vtkwriter in the sequence writer (Fixes
    [#77])
-   [x] Create API for time stepping data, so that I don't have to always
    start from 0 (e.g. in case of a restart)
-   [x] Test restart of VTKSequenceWriter
-   [x] Add entry in changelog

Make VTKSequenceWriter suitable for restarted simulations, as discussed by the
IO group on the [Dune dev meeting 2018]

See merge request [!319]

  [#77]: gitlab.dune-project.org/NoneNone/issues/77
  [Dune dev meeting 2018]: https://www.dune-project.org/community/meetings/2018-11-devmeeting/#vtk
  [!319]: gitlab.dune-project.org/core/dune-grid/merge_requests/319


Closes #77