Calling Graph.reverse() on a graph with a 2-cycle throws an Error

[DCtheTall]

Graph.reverse() is currently broken for directed graphs with a 2-cycle. Consider the complete, directed graph of size 2, if you call the reverse() method in its current form, it will throw an error.

I think a fix would be to make the reverse() methods of both the Graph and GraphEdge objects pure functions which return new instances of the respective objects.

This would add the additional benefit of allowing calling these functions to not change the original graph, which can be overwritten/garbage collected if desired.

I am currently working on a pull request for issue #252 and would like to use the already-implemented stronglyConnectedComponents function to implement Johnson's algorithm.

I would be willing to open a preliminary PR making the change described above (and refactoring tests) if the owners are ok with this change.

[DCtheTall]

Bumping this issue, is this a change that the owners of this repo would accept? If so I can open a PR for this within a few days.