- the insert_point_on_segment method in the Segment_Voronoi_diagram_2
  class has a bug: it does not handle the case where ff1 or ff2 are
  infinite faces.
- make the storage site take a point container as template parameter,
  rather than a point handle
- make sure all types in the doc correspond to real types in the code
- add a container that stores the input sites 
- add the methods number_of_{input,output}_sites() in the
  Segment_Voronoi_diagram_2 class.
- add swap method in hierarchy; verify that the swap method in the segment
  Voronoi diagram class works correctly (does it swap the point container
  correctly?)
- make the intersection tag in the traits to be the first template argument,
  thus no default value
- change the return type of the do_intersect predicate (and potentially
  its name) to be a char instead of std::pair<int,int>; the char should
  encode the type of arrangement; actually rename do_intersect to 
  arrangement_type and return an enumeration type
- in class Svd_basic_constructions_C2.h, inside method to_ft, check
  if sqrt is supported by the number type; actually this is ok for predicates
  but constructions also need to use this too.
- change the doc so that the requirement for the face base is the
  Triangulation_face_base_2 class
- add the methods insert_polyline and insert_polygon that do some
  optimized insertion for polylines and polygons; add these in both
  the one-level and hierarchy classes
- add the check for a segment intersecting an existing point and
  a point intersecting an existing segment.