Find the value of a maximum singlecommodity flow.
Parameters :  G : NetworkX graph
s : node
t : node
capacity: string :


Returns :  flow_value : integer, float

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 = nx.max_flow(G, 'x', 'y')
>>> flow
3.0