[Nauty] canonical label for directed graphs

[Sterten at aol.com]

the newest version also allows you to label directed graphs, but it could  be 
slow
for some graphs with many vertices.
You can convert a directed graph into a bipartite undirected graph with  twice
as many vertices and this could be faster for labeling.
We had discussed this here some time ago.
It should be in the archive.
 
cheers, Guenter.