[bugfix] Take normalized face normals into account when calculating tpfa.

At least for Cpgrid the face normal is normalized which has to be taken into
account in tpfa_htrans_compute.
We therefore multiply the facenormal with the face area in cases (read Cpgrid)
where this needed.

opm/core/grid/GridHelpers.cpp

@@ -58,6 +58,11 @@ const double* faceNormal(const UnstructuredGrid& grid, int face_index)
     return grid.face_normals+face_index*grid.dimensions;
 }
 
+double faceArea(const UnstructuredGrid& grid, int face_index)
+{
+    return grid.face_areas[face_index];
+}
+
 SparseTableView cell2Faces(const UnstructuredGrid& grid)
 {
     return SparseTableView(grid.cell_faces, grid.cell_facepos, numCells(grid));