static void outputStateVtk(const UnstructuredGrid& grid,
                               const Opm::BlackoilState& state,
                               const int step,
                               const std::string& output_dir)
    {
        // Write data in VTK format.
        std::ostringstream vtkfilename;
        vtkfilename << output_dir << "/vtk_files";
        boost::filesystem::path fpath(vtkfilename.str());
        try {
          create_directories(fpath);
        }
        catch (...) {
          OPM_THROW(std::runtime_error, "Creating directories failed: " << fpath);
        }
        vtkfilename << "/output-" << std::setw(3) << std::setfill('0') << step << ".vtu";
        std::ofstream vtkfile(vtkfilename.str().c_str());
        if (!vtkfile) {
            OPM_THROW(std::runtime_error, "Failed to open " << vtkfilename.str());
        }

        Opm::DataMap dm;
        dm["saturation"] = &state.saturation();
        dm["pressure"] = &state.pressure();
        std::vector<double> cell_velocity;
        Opm::estimateCellVelocity(grid, state.faceflux(), cell_velocity);
        dm["velocity"] = &cell_velocity;
        Opm::writeVtkData(grid, dm, vtkfile);
    }
    static void outputStateMatlab(const UnstructuredGrid& grid,
                                  const Opm::BlackoilState& state,
                                  const int step,
                                  const std::string& output_dir)
    {
        Opm::DataMap dm;
        dm["saturation"] = &state.saturation();
        dm["pressure"] = &state.pressure();
        dm["surfvolume"] = &state.surfacevol();
        std::vector<double> cell_velocity;
        Opm::estimateCellVelocity(grid, state.faceflux(), cell_velocity);
        dm["velocity"] = &cell_velocity;

        // Write data (not grid) in Matlab format
        for (Opm::DataMap::const_iterator it = dm.begin(); it != dm.end(); ++it) {
            std::ostringstream fname;
            fname << output_dir << "/" << it->first;
            boost::filesystem::path fpath = fname.str();
            try {
              create_directories(fpath);
            }
            catch (...) {
              OPM_THROW(std::runtime_error, "Creating directories failed: " << fpath);
            }
            fname << "/" << std::setw(3) << std::setfill('0') << step << ".txt";
            std::ofstream file(fname.str().c_str());
            if (!file) {
                OPM_THROW(std::runtime_error, "Failed to open " << fname.str());
            }
            file.precision(15);
            const std::vector<double>& d = *(it->second);
            std::copy(d.begin(), d.end(), std::ostream_iterator<double>(file, "\n"));
        }
    }
    void outputStateVtk(const Dune::CpGrid& grid,
                        const Opm::BlackoilState& state,
                        const int step,
                        const std::string& output_dir)
    {
        // Write data in VTK format.
        std::ostringstream vtkfilename;
        std::ostringstream vtkpath;
        vtkpath << output_dir << "/vtk_files";
        vtkpath << "/output-" << std::setw(3) << std::setfill('0') << step;
        boost::filesystem::path fpath(vtkpath.str());
        try {
            create_directories(fpath);
        }
        catch (...) {
            OPM_THROW(std::runtime_error, "Creating directories failed: " << fpath);
        }
        vtkfilename << "output-" << std::setw(3) << std::setfill('0') << step;
#if DUNE_VERSION_NEWER(DUNE_GRID, 2, 3)
        Dune::VTKWriter<Dune::CpGrid::LeafGridView> writer(grid.leafGridView(), Dune::VTK::nonconforming);
#else
        Dune::VTKWriter<Dune::CpGrid::LeafGridView> writer(grid.leafView(), Dune::VTK::nonconforming);
#endif
        writer.addCellData(state.saturation(), "saturation", state.numPhases());
        writer.addCellData(state.pressure(), "pressure", 1);
        
        std::vector<double> cell_velocity;
        Opm::estimateCellVelocity(AutoDiffGrid::numCells(grid),
                                  AutoDiffGrid::numFaces(grid),
                                  AutoDiffGrid::beginFaceCentroids(grid),
                                  AutoDiffGrid::faceCells(grid),
                                  AutoDiffGrid::beginCellCentroids(grid),
                                  AutoDiffGrid::beginCellVolumes(grid),
                                  AutoDiffGrid::dimensions(grid),
                                  state.faceflux(), cell_velocity);
        writer.addCellData(cell_velocity, "velocity", Dune::CpGrid::dimension);
        writer.pwrite(vtkfilename.str(), vtkpath.str(), std::string("."), Dune::VTK::ascii);
    }