Add cluster_selection_epsilon parameter

[JelmerBot]

Thanks for this repository! This pull request implements the _cluster_selectionepsilon feature, which I have found useful in the past. I tried to port the previous implementation's relevant parts to numba without changing its structure too much. However, there is one notable difference: I stopped the _allow_singlecluster flag from affecting the epsilon threshold.

Let me know if things need to change for this pull request to be accpeted. I'll try to follow up on your comments!

[lmcinnes]

Looks good to me. Thanks for this -- as you say, the epsilon selection can be quite useful at times.