Merge branch 'fix/boxnetworkgridnormalcomputation' into 'master'

Fix/box normal vector computation on 2d network grids

The normal was not scaled by the right area for 2d network grids in 3d space,
which led to wrong fluxes.

See merge request !288

dumux/implicit/box/fvelementgeometry.hh

@@ -943,6 +943,10 @@ public:
                 this->crossProduct(elemNormal, elemVec1, elemVec2);
                 this->crossProduct(scvFace.normal, faceVec, elemNormal);
 
+                // scale normal vector by the area
+                scvFace.normal /= scvFace.normal.two_norm();
+                scvFace.normal *= faceVec.two_norm();
+
                 const auto diffVec = subContVol[j].global - subContVol[i].global;
                 // make sure the normal points to the right direction
                 if (scvFace.normal * diffVec < 0)