Undirected graph documentation and concept refinements.
* quite a few bug fixes
* concept::UndirGraph is almost complete and looks quite good.
1 ./preflow_demo_leda < $1
2 ./preflow_demo_boost < $1
3 ./preflow_demo_jacint < $1
4 ./preflow_demo_athos < $1
5 ./edmonds_karp_demo < $1
6 ./edmonds_karp_demo_alpar < $1
7 ./edmonds_karp_demo_boost < $1