cleanup of source code, removed several warnings

dune/vtk/datacollectors/continuousdatacollector.hh

dune/vtk/datacollectors/discontinuousdatacollector.hh

dune/vtk/datacollectors/quadraticdatacollector.hh

dune/vtk/datacollectors/spdatacollector.hh

dune/vtk/datacollectors/structureddatacollector.hh

dune/vtk/datacollectors/yaspdatacollector.hh

dune/vtk/gridcreator.hh

dune/vtk/vtkreader.impl.hh

dune/vtk/writers/vtkimagedatawriter.impl.hh

dune/vtk/writers/vtkrectilineargridwriter.impl.hh

dune/vtk/writers/vtkstructuredgridwriter.impl.hh

dune/vtk/writers/vtkunstructuredgridwriter.hh

dune/vtk/writers/vtkunstructuredgridwriter.impl.hh

dune/vtk/writers/vtkwriterinterface.hh

dune/vtk/writers/vtkwriterinterface.impl.hh