Move everything into the Vtk namespace

Summary

This MR cleans up the implementation. All classes are moved to the Dune::Vtk namespace and some classes are renamed:

• VtkImageDataWriter -> Vtk::ImageDataWriter (moved into writers/imagedatawriter.hh)
• VtkRectilinearGridWriter -> Vtk::RectilinearGridWriter (moved into writers/rectilineargridwriter.hh)
• VtkStructuredGridWriter -> Vtk::StructuredGridWriter (moved into writers/structuredgridwriter.hh)
• VtkUnstructuredGridWriter -> Vtk::UnstructuredGridWriter (moved into writers/unstructuredgridwriter.hh)
• VtkWriterInterface -> Vtk::VtkWriterBase (moved into vtkwriterbase.hh)
• VtkTimeseriesWriter -> Vtk::TimeseriesWriter (moved into timeserieswriter.hh)
• VtkReader -> Vtk::VtkReader (*)
• VtkWriter -> Vtk::VtkWriter (*)
• PvdWriter -> Vtk::PvdWriter (*)
• VtkError -> Vtk::VtkError (*)

The old classes are transformed into aliases to the new classes with a deprecation warning, and the old files include these aliases instead of the implementation, have the include of the new file and a deprecation warning.

(*) The names in the Dune namespace are the same as those in the Dune::Vtk. This might introduce ambiguities if you are using namespace Dune::Vtk in combination with using namespace Dune. Then the unqualified names are ambiguous. Don't know how to circumvent this except for introducing different names in the Vtk namespace.