moved some interface files to root directory

dune/vtk/CMakeLists.txt

dune/vtk/datacollectorinterface.hh

dune/vtk/datacollectorinterface.impl.hh

dune/vtk/datacollectors/CMakeLists.txt

dune/vtk/datacollectors/structureddatacollector.hh

dune/vtk/datacollectors/unstructureddatacollector.hh

dune/vtk/vtkwriter.hh

dune/vtk/vtkwriterinterface.hh

dune/vtk/vtkwriterinterface.impl.hh

dune/vtk/writers/CMakeLists.txt

dune/vtk/writers/vtkimagedatawriter.hh

dune/vtk/writers/vtkrectilineargridwriter.hh

dune/vtk/writers/vtkstructuredgridwriter.hh

dune/vtk/writers/vtkunstructuredgridwriter.hh

src/structuredgridwriter.cc