Bidirectional Dijkstra

[gdalle]

I think @Dolgalad has an implementation?

[Dolgalad]

Created Pr #268 for your review, I'm still planning on adding tests and some documentation

[gdalle]

Awesome, thank you! Don't hesitate to ping me when you do (no rush)

[Dolgalad]

I've added a simple test and some documentation. Information about the bidirectional Dijkstra is a bit lacking online but I found Matthew Towers' blog post to be quite clear (https://www.homepages.ucl.ac.uk/~ucahmto/math/2020/05/30/bidirectional-dijkstra.html)