- all the code is factored around visit_incident_cells.
- incident_edges has been added
- finite versions of all incident_*(vertex) have been added
thanks to a Filter template parameter
- Add an undocumented circumcenter() template member function
in Triangulation_ds_cell_base_3 so that we can continue to use it as default
template parameter of the TDS, and still have .dual() working when only the
VertexBase type is changed (e.g. for the hierarchy).