graph_io under construction
This is a working version, but needs more improvments.

todo:
documention + fix the file format
improve the exception system

add some possible asserts

tutorials

 ./preflow_demo_leda < $1

 ./preflow_demo_boost < $1

 ./preflow_demo_jacint < $1

 ./preflow_demo_athos < $1

 ./edmonds_karp_demo < $1

 ./edmonds_karp_demo_alpar < $1

 ./edmonds_karp_demo_boost < $1