Add generic BFS / DFS traversal code that takes visitor structures

[sbromberger]

In the old days, we had generic BFS / DFS code that you could pass visitor functions to, and you could then create your own traversal algorithms (like gdistances, for example) that would use BFS or DFS with custom state. Turns out in pre-1.0, this was very slow so we opted to create custom BFS implementations for specific functions.

@jpfairbanks suggests that performance may be improved post 1.0. It would be nice to have some code to benchmark. Adding the relevant tags so that GSOC candidates might see this as a good opportunity to get familiar w/ LightGraphs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.