Implement construction of cell-cell connectivity by nodes

It is just a test and should be plugged and cleaned-up

src/mesh/Connectivity.hpp

@@ -550,12 +550,13 @@ class Connectivity final
   CSRGraph cellToCellGraph() const
   {
     std::vector<std::set<int>> cell_cells(this->numberOfCells());
+    if constexpr (true) {
       const auto& face_to_cell_matrix
           = this->faceToCellMatrix();
 
       for (FaceId l=0; l<this->numberOfFaces(); ++l) {
         const auto& face_to_cell = face_to_cell_matrix[l];
-      if (face_to_cell.size() == 2) {
+        if (face_to_cell.size() > 1) {
           const CellId cell_0 = face_to_cell[0];
           const CellId cell_1 = face_to_cell[1];
 
@@ -563,6 +564,22 @@ class Connectivity final
           cell_cells[cell_1].insert(cell_0);
         }
       }
+    } else {
+      const auto& node_to_cell_matrix
+          = this->nodeToCellMatrix();
+
+      for (NodeId l=0; l<this->numberOfNodes(); ++l) {
+        const auto& node_to_cell = node_to_cell_matrix[l];
+        for (size_t i_cell=0; i_cell<node_to_cell.size(); ++i_cell) {
+          const CellId cell_0 = node_to_cell[i_cell];
+          for (size_t j_cell=0; j_cell<i_cell; ++j_cell) {
+            const CellId cell_1 = node_to_cell[j_cell];
+            cell_cells[cell_0].insert(cell_1);
+            cell_cells[cell_1].insert(cell_0);
+          }
+        }
+      }
+    }
 
     Array<int> entries(this->numberOfCells()+1);
     entries[0]=0;