Cosine distance RP-Trees

[lmcinnes]

UMAP currently uses RP-trees to initialise the NN-descent algorithm. The current version of this uses euclidean distance RP-Trees. In principle cosine distance RP-trees are simple to implement and would be more useful for cosine and correlation distance metrics. Allowing the option would be beneficial.

This requires both implementing the trees (simply write a new splitting function), and threading the option through the code to be able to present it to the user an class instantiation time.