Faster cluster selection

[markopy]

This PR makes performance improvements to epsilon_search and get_cluster_tree_leaves by optimizing Cython compilation and exploiting how the tree is stored.

The epsilon_search improvements are for eom selection.

For leaf selection epsilon_search is no longer used and the epsilon criteria is checked during get_cluster_tree_leaves. This avoids traversing the tree down to the leaves and then finding the leaves again to traverse up.

[pep8speaks]

Hello @markopy, Thank you for submitting the Pull Request !

• In the file hdbscan/flat.py:

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