replace Dune::Std::optional with std::optional

dune/vtk/filewriter.hh

dune/vtk/gridcreators/lagrangegridcreator.hh

dune/vtk/pvdwriter.hh

dune/vtk/pvdwriter.impl.hh

dune/vtk/vtkfunction.hh

dune/vtk/vtktimeserieswriter.hh

dune/vtk/vtktimeserieswriter.impl.hh

dune/vtk/vtkwriterinterface.hh

dune/vtk/vtkwriterinterface.impl.hh

dune/vtk/writers/vtkrectilineargridwriter.hh

dune/vtk/writers/vtkrectilineargridwriter.impl.hh

dune/vtk/writers/vtkunstructuredgridwriter.hh

dune/vtk/writers/vtkunstructuredgridwriter.impl.hh