Restructuring of vtk writers to remove redundency

dune/vtk/vtktimeserieswriter.hh

dune/vtk/vtktimeserieswriter.impl.hh

dune/vtk/vtkwriterinterface.hh

dune/vtk/vtkwriterinterface.impl.hh

dune/vtk/writers/vtkimagedatawriter.hh

dune/vtk/writers/vtkimagedatawriter.impl.hh

dune/vtk/writers/vtkrectilineargridwriter.hh

dune/vtk/writers/vtkrectilineargridwriter.impl.hh

dune/vtk/writers/vtkstructuredgridwriter.hh

dune/vtk/writers/vtkstructuredgridwriter.impl.hh

dune/vtk/writers/vtkunstructuredgridwriter.hh

dune/vtk/writers/vtkunstructuredgridwriter.impl.hh