+These groups are named according to the position of the group in the list:
+the group j_n_p is the group of the flat elements that are built between the group #n and the group #p in the list.
+If there is no shared faces between the group #n and the group #p in the list, the group j_n_p is not created.
+All the flat elements are gathered into the group named "joints3D" (or "joints2D" in 2D situation).
+The flat element of the multiple junctions between the simple junction are stored in a group named "jointsMultiples".