Graph and UndirGraph concept modifications.

* For incidence iterators ({In,Out,Inc}EdgeIt) there is now baseNode and
runningNode functions in graph interface
* For Edge in undir graphs: Edge(UndirGraph const &, UndirEdge, Node)
constructor. Same for IncEdgeIt
* Edge(UndirEdge, bool) constructor is no more in the public interface. (But we
need it in the developpers interface).