weighted_circumcenter() replaces circumcenter() in the regular framework,
which is not a member of TriangulationCellBase_3 anymore.
Triangulation_cell_base_with_circumcenter_3 is deprecated and replaced by:
Delaunay_triangulation_cell_base_with_circumcenter_3
and
Regular_triangulation_cell_base_with_circumcenter_3.
Implement the changes in documentation, Triangulation_3 code and Mesh_3 code.
Jane Tournois