Add corners to the second member for Neumann boundary conditions

src/scheme/ScalarNodalScheme.cpp

@@ -615,9 +615,13 @@ class ScalarNodalSchemeHandler::ScalarNodalScheme : public ScalarNodalSchemeHand
             }
           } else if (node_is_corner[node_id]) {
             const auto& node_to_face = node_to_face_matrix[node_id];
+            const CellId cell_id     = node_to_cell[0];
+            double sum_mes_l         = 0;
             for (size_t i_face = 0; i_face < node_to_face.size(); ++i_face) {
               FaceId face_id = node_to_face[i_face];
+              sum_mes_l += mes_l[face_id];
             }
+            b[cell_id] += 0.5 * node_boundary_values[node_id] * sum_mes_l;
           }
         };