[CDE] adapt CCFV operator for rediscretization in AMG-DG

... on non axi-parallel hexahedral / quadrilateral meshes.

dune/pdelab/localoperator/convectiondiffusionccfv.hh

@@ -258,7 +258,8 @@ namespace Dune {
 
         // distance between the two cell centers
         global_inside -= global_outside;
-        auto distance = global_inside.two_norm();
+        // auto distance = global_inside.two_norm();
+        auto distance = std::min(geo_inside.volume(),geo_outside.volume())/geo.volume();
 
         // contribution to jacobians on inside element and outside element for test and trial function
         mat_ss.accumulate(lfsu_s,0,lfsu_s,0,   k_avg*face_volume/distance );
@@ -442,11 +443,15 @@ namespace Dune {
             auto global_inside = geo_inside.global(local_inside);
             auto global_outside = geo.global(face_local);
             global_inside -= global_outside;
-            auto distance = global_inside.two_norm();
-            // Calculate distance to center of neighbouring boundary cell, to
-            // be consistent with treatment in DG operator
-            if (use_for_amg4dg_rediscretisation)
-              distance *= 2.0;
+            // auto distance = global_inside.two_norm();
+            // // Calculate distance to center of neighbouring boundary cell, to
+            // // be consistent with treatment in DG operator
+            // if (use_for_amg4dg_rediscretisation)
+            //   distance *= 2.0;
+
+            auto distance = geo_inside.volume()/geo.volume();
+            if (not use_for_amg4dg_rediscretisation)
+              distance *= 0.5;
 
             // evaluate diffusion coefficient
             auto tensor_inside = param.A(cell_inside,local_inside);