Floyd Warshall on Undirected Graph

[privateOmega]

@cpettitt Thank you so much for creating this library, I love it for the easiness in use. I have been using this library in a project of mine for some time now.

I encountered a situation now which requires me to apply floyd-warshall on a graph, but sadly the implementation in this library works only on directed graphs as it iterates through edges to find the distance and predecessor. I would really appreciate it, if someone could add support for undirected graphs as well. I think this could be solved by adding the respective inverse edge also to the edgeList, but I feel I shouldn't do it because it goes against concept of undirected graph in this library.