implement edge.to_tuple()

[akelleh]

Given an edge, we want access to the tuple (parent_node_name, child_node_name) so we can write the edgelist for the graph. the method of the Edge object called to_tuple() should return the tuple listed above, so that we could run , for example
edgelist = [edge.to_tuple() for edge in g.edges()]
for some Graph object g to get the edgelist in the standard format [(1,2),(2,3),...] for integer named edges.