remove the central forward declaration

dune/vtk/CMakeLists.txt

dune/vtk/datacollectorinterface.hh

dune/vtk/datacollectors/continuousdatacollector.hh

dune/vtk/datacollectors/discontinuousdatacollector.hh

dune/vtk/datacollectors/lagrangedatacollector.hh

dune/vtk/datacollectors/quadraticdatacollector.hh

dune/vtk/datacollectors/spdatacollector.hh

dune/vtk/datacollectors/structureddatacollector.hh

dune/vtk/datacollectors/unstructureddatacollector.hh

dune/vtk/datacollectors/yaspdatacollector.hh

dune/vtk/filereader.hh

dune/vtk/filewriter.hh

dune/vtk/forward.hh

dune/vtk/gridcreatorinterface.hh

dune/vtk/gridcreators/common.hh

dune/vtk/gridcreators/derivedgridcreator.hh

dune/vtk/gridcreators/discontinuousgridcreator.hh

dune/vtk/gridcreators/lagrangegridcreator.hh

dune/vtk/gridcreators/parallelgridcreator.hh

dune/vtk/gridcreators/serialgridcreator.hh