Compute the value of a minimum (s, t)cut.
Use the maxflow mincut theorem, i.e., the capacity of a minimum capacity cut is equal to the flow value of a maximum flow.
Parameters :  G : NetworkX graph
s : node
t : node
capacity: string :


Returns :  cutValue : integer, float

Raises :  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)
>>> nx.min_cut(G, 'x', 'y')
3.0