Use a predicate that discards tensors from vtk output

dune/perftool/vtkpredicate.hh

+#ifndef DUNE_PERFTOOL_VTKPREDICATE_HH
+#define DUNE_PERFTOOL_VTKPREDICATE_HH
+
+/** A predicate for vtk output
+ *  that cuts all vector grid function spaces with a length greater than 4.
+ *  This is necessary because VTK cannot handle them.
+ */
+struct CuttingPredicate
+{
+  template<typename T, typename TP>
+  bool operator()(const T& t, TP tp) const
+  {
+    return Dune::TypeTree::degree(t) < 4;
+  }
+};
+
+#endif
\ No newline at end of file

python/dune/perftool/pdelab/driver.py

@@ -901,6 +901,24 @@ def define_gfs_name(element, prefix=()):
     return "{}.name(\"data_{}\");".format(gfs, "_".join(str(p) for p in prefix))
 
 
+@symbol
+def type_predicate():
+    include_file("dune/perftool/vtkpredicate.hh", filetag="driver")
+    return "CuttingPredicate"
+
+
+@preamble
+def define_predicate(name):
+    t = type_predicate()
+    return "{} {};".format(t, name)
+
+
+@symbol
+def name_predicate():
+    define_predicate("predicate")
+    return "predicate"
+
+
 @preamble
 def vtkoutput():
     element = _form.coefficients()[0].ufl_element()
@@ -910,8 +928,9 @@ def vtkoutput():
     gfs = name_gfs(element)
     vec = name_vector()
     vtkfile = name_vtkfile()
+    predicate = name_predicate()
     dune_solve()
     print_residual()
     print_matrix()
-    return ["Dune::PDELab::addSolutionToVTKWriter({}, {}, {});".format(vtkwriter, gfs, vec),
+    return ["Dune::PDELab::addSolutionToVTKWriter({}, {}, {}, Dune::PDELab::vtk::defaultNameScheme(), {});".format(vtkwriter, gfs, vec, predicate),
             "{}.write({}, Dune::VTK::ascii);".format(vtkwriter, vtkfile)]