Drop nmslib? Support other approximate MST libs? ArborX?

[gagolews]

The nmslib package hasn't been updated awhile... :/

https://pypi.org/project/nmslib/

https://github.com/nmslib/nmslib

[gagolews]

https://github.com/arborx/ArborX/blob/master/src/cluster/ArborX_MinimumSpanningTree.hpp ?

[gagolews]

#90 Own Approximate MST based on K-means/medoids