[bugfix] fix wrong factor in problem mmdg6

dune/mmdg/problems/mmdgproblem6.hh

@@ -61,8 +61,8 @@ public:
     const Scalar hyperbolicFactor =
       (coshsqrt2 * coshsqrt2 - (2 * xi_ - 1) * sinhsqrt2 * sinhsqrt2);
 
-    return -48.0 * d0_ * d0_ * std::exp(-8.0 * pos[1]) * hyperbolicFactor * ( cos4y
-      + 3.0 * std::sin(4.0 * pos[1]) ) - sqrt2 * std::sinh(sqrt2) * cos4y;
+    return -16.0 * d0_ * d0_ * std::exp(-8.0 * pos[1]) * hyperbolicFactor * ( 
+      cos4y + 3.0 * std::sin(4.0 * pos[1]) ) - sqrt2 * std::sinh(sqrt2) * cos4y;
   }
 
   //aperture d of the fracture at position pos