adapt problem mmdg5 to have a non-constant interface solution

dune/mmdg/problems/mmdgproblem5.hh

@@ -20,13 +20,13 @@ public:
   Scalar exactSolution (const Coordinate& pos) const
   {
     return (pos[1] > pos[0]) ?
-      1.0 / (1.0 + pos[0] * pos[0]) : -1.0 / (1.0 + pos[1] * pos[1]);
+      3.0 / (1.0 + pos[0] * pos[0]) : -1.0 / (1.0 + pos[1] * pos[1]);
   }
 
   //the exact solution on the interface at position pos
   Scalar exactInterfaceSolution (const Coordinate& pos) const
   {
-    return 0.0;
+    return 0.5 / (1.0 + pos[1] * pos[1]);
   }
 
   //indicates whether an exact solution is implemented for the problem
@@ -42,13 +42,13 @@ public:
     const Scalar y2Plus1 = pos[1] * pos[1] + 1.0;
 
     return (pos[1] > pos[0]) ?
-      -4.0 * pos[0] / (x2Plus1 * x2Plus1) : 4.0 * pos[1] / (y2Plus1 * y2Plus1);
+      -12.0 * pos[0] / (x2Plus1 * x2Plus1) : 4.0 * pos[1] / (y2Plus1 * y2Plus1);
   }
 
   //interface source term at position pos
   Scalar qInterface (const Coordinate& pos) const
   {
-    return 0.0;
+    return 0.5 * d_ * d_ - 2.0 * sqrt(2.0) / (1.0 + pos[1] * pos[1]);
   }
 
   //aperture d of the fracture at position pos