Make this package fast and customisable enough to be used by Vole, GraphBacktracking, ferret...

[wilfwilson]

So far, this package has not been focused on performance. (Which is entirely reasonable). But there are several packages which have different implementations of constructing orbital graphs, and it would be nice if they could use this package instead (especially if we can beat the performance of those packages).

I don't include any specifics here (although perhaps I will add details), but for now I include it as a goal.

This issue can be closed once OrbitalGraphs is used by GraphBacktracking to construct its orbital graphs, or if we decide to give up on this goal.

Related issues: #27, #33, #34, #35