octree to search for the nearest neighbor

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hello,

Thanks for your library. It is very great.
I have a question concerning to octree (that I didn't find an answer in the documentation). Can we use octree to find the nearest neighbor of a point (with coordinate (x, y, z)). I know that in open3d, we use kdtree (knn) to search for a nearest neighbor. But I think it is faster in octree than in kdtree (in particular for the big point cloud).

Many thanks.