shift evaluation for vtk output to correctly picture discontinuities at the interface

dune/mmdg/dg.hh

@@ -524,8 +524,10 @@ protected:
       for (int k = 0; k < geo.corners(); k++)
       {
         if (problem_.hasExactSolution())
-        {
-          exactPressure[elemIdxSLE + k] = problem_.exactSolution(geo.corner(k));
+        { //evaluation at the corners is slightly shifted towards the center of
+          //the element to correctly picture discontinuities of the solution
+          exactPressure[elemIdxSLE + k] = problem_.exactSolution(
+            0.9999 * geo.corner(k) + 0.0001 * geo.center());
         }
 
         //contribution of the basis function