insert_dangling_cell_1_in_cell_2 ( CMap& cm, typename CMap::Dart_handle dh)
Inserts a 1-cell in a the 2-cell containing dh, the 1-cell
being attached only by one of its extremity to the 0-cell containing dh.
Returns a handle on one dart belonging to the new 1-cell.
Precondition:
CMap::dimension≥2 and *dh∈cm.darts().
See example in Figure 27.13.
