fixed problem with the way the edges were being numbered when solving ta problem on networks. The symmetric braess network now works as it should. i.e. It is now correct as it was in the flow_graph_script. I think it had to do with the way g.edges_iter() works. Also added a function to reassign the 'b' parameters on an already existing network. FIxed som things to make working with networks that have no predetermined origin and dest nodes easier