[dune-pdelab] Very large .vtu files

[conf86 at web.de]

Sorry for my late response.
Thank you, I did it just like that. 
If someone is interested, this is how I solved it:

 // <<<9>>> time loop
  U unew(gfs,0.0);                                            // solution to be computed
  while (time<tend-1e-8) {
      // do time step
      bctype.setTime(time+dt);                                // compute constraints
      cc.clear();                                             // for this time step
      Dune::PDELab::constraints(bctype,gfs,cc);
      osm.apply(time,dt,uold,g,unew);                         // do one time step
      
      if (fmod(time,10*dt)<=1e-3*dt){                         // write every 10th step into a graphic file
      // graphics
      Dune::SubsamplingVTKWriter<GV> vtkwriter(gv,3);
      Dune::PDELab::addSolutionToVTKWriter(vtkwriter,gfs,unew);
      vtkwriter.write(fn.getName(),Dune::VTK::appendedraw);
      fn.increment();
      }

      uold = unew;                                            // advance time step
      time += dt;
    }