Allow setting cluster_selection_epsilon in hdbscan() about dbscan

[mhahsler]

@joeroe Do you know how to add this functionality? @peekxc is the original author of the HDBSCAN implementation.

[peekxc]

I'm not familiar with the paper mentioned. That being said, it may be possible to post-process the info returned by the internal hdbscan calls to add this. After all, obtaining DBSCAN* clusters amounts to cutting linkage produced by hdbscan via cutree.

Otherwise, the computeStability function would have to be forked on the cpp-side, which is a bit involved.

[joeroe]

It's a bit beyond me, to be honest. Since it's a modification to the cluster extraction algorithm, does it have to be in the Cpp code? Here's the PR that added it to Python's hdbscan: scikit-learn-contrib/hdbscan#329. Perhaps @cmalzer or @lmcinnes could help?

[lmcinnes]

My R is not great, and tracing it into the C++ I lose track a little of exactly what data structures we have, but if I read it tight you should be able to insert code here at the R level to do the epsilon checks and pick different clusters accordingly. There are different data structures in play; I think this is a lot easier if you have the condensed (in this package simplified) tree data structure available -- it isn't clear to me if that's actually exposed at the R level, or if you have to be in the C++.