Find the the longest-lived hierarchy of a dendrogram.
get_longevity_cut_height(dend, max_height = max(cophenetic(dend)))
The point just above the merge height with the longest time to the next merge point.
A dendrogram.
The maximum height of the dendrogram; if this is not provided the last merge height of the input dendrogram will be used, which will make cutting to one cluster impossible!