Use BallTree for nearest neighbor search

[blaylockbk]

This would replace or expand the current herbie accessor herbie.accessors.nearest_points.

• BallTree: https://scikit-learn.org/stable/modules/generated/sklearn.neighbors.BallTree.html
• Example: https://gist.github.com/benbovy/adcb7d3b14d22014444a2c57efb30696
• Discussion: https://github.com/pydata/xarray/discussions/5009
• https://github.com/benbovy/pys2index
• Probably implement this xarray extension: https://github.com/xarray-contrib/xoak