Find a maximum singlecommodity flow using the FordFulkerson algorithm.
This algorithm uses EdmondsKarpDinitz path selection rule which guarantees a running time of O(nm^2) for n nodes and m edges.
Parameters :  G : NetworkX graph
s : node
t : node
capacity: string :


Returns :  flow_value : integer, float
flow_dict : dictionary

Raises :  NetworkXError :
NetworkXUnbounded :

Examples
>>> import networkx as nx
>>> G = nx.DiGraph()
>>> G.add_edge('x','a', capacity=3.0)
>>> G.add_edge('x','b', capacity=1.0)
>>> G.add_edge('a','c', capacity=3.0)
>>> G.add_edge('b','c', capacity=5.0)
>>> G.add_edge('b','d', capacity=4.0)
>>> G.add_edge('d','e', capacity=2.0)
>>> G.add_edge('c','y', capacity=2.0)
>>> G.add_edge('e','y', capacity=3.0)
>>> flow, F = nx.ford_fulkerson(G, 'x', 'y')
>>> flow
3.0