Working with nx.graph objects when there are disconnected nodes in the graph

[jernsting]

If you have a nx.graph object where a node is disconnected (or removed) from the graph, FINDER will fail to analyze this.

Code for reproduction:

>>> test = np.array([0, 0, 0, 1], [0, 0, 0, 1], [0, 0, 0, 1], [1, 1, 1, 0]])
>>> g = nx.from_numpy_array(test)
>>> g.remove_node(2)
>>> 
>>> # some more detail:
>>> a, b = zip(*g.edges())
>>> a
(0, 1)
>>> b    # <<< Here is the error!
(3, 3)
>>> len(g.nodes())
3

I already created a pull request (#12) on this and I am only filing this issue for convenient issue tracking.