Fix bug in path_lengths

[alexalekseyenko]

As it's written, path_lengths returns all distances, including the one from the source to itself. Assuming that shouldn't be the case, as the solutions indicate the average path length should be 1.

[alexalekseyenko]

If it isn't too unreadable, probably better to do it this way(?):

def path_lengths(G):
    length_iter = nx.shortest_path_length(G)
    for source, dist_map in length_iter:
        for dest in set(dist_map.keys()).difference([source]):
            yield dist_map[dest]