Non edges enumeration

[kmkurn]

Here is the implementation of non-existent edges enumeration as discussed in #1117. With this, one can perform link prediction algorithm to a graph by something like:

import networkx as nx

def linkpred(graph):
    for (u, v) in nx.non_edges(graph):
        p = nx.common_neighbors(graph, u, v)
        yield (u, v, p)

Please let me know what you think.

[ysitu]

Use assert_equal and friends.

[ysitu]

Save for the comments above this otherwise looks good to me.

[chebee7i]

We can merge this once those changes are done.