Speed up nearest neighbor search

[dansbits]

@carlzimmerman Brought up the fact that the nearest neighbor search used for KNNClassifier and KNNRegression could be sped up. Currently it does a full scan of all points in the training set.

Some potential options for speeding this up is to store training data in a KD tree or a ball tree.

Introduction to KD trees:
https://www.analyticsvidhya.com/blog/2017/11/information-retrieval-using-kdtree/

Introduction to kd trees and ball trees:
https://ashokharnal.wordpress.com/tag/ball-tree-explained-in-simple-manner/