`nearest_neighbour*` naming

[tombh]

Just a minor issue, I mention it in case anybody else suffers the same. I lost a few hours struggling with nearest_neighbour() not returning enough results. In hindsight I did actually see Clippy telling me that iterating over an Option is not a good idea, but I was too convinced that the lack of results in my for loop was caused by another bug in my code. Then finally I saw nearest_neighbours() in my editor's autocomplete and all was well in the world (I actually ended up with nearest_neighbour_iter(), but that's digressing).

I wonder if enabling the clippy::similar_names lint would have helped with this? What would even be a "better" naming? nearest_neighbour_first and nearest_neighbours?

Like I say it's minor and just documenting it for future searches.

Thanks for the great work ✨