GitLab

* add submodule nanoflann

* finish python API for knn

* finish ndarray adaptor

* finish cpu-kdtree version of knn

* use openmp

* add endline

* upt

* upt

* fix format and code style

* upt

* add warning for gpu-cpu copy

* avoid contiguous copy
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>
Co-authored-by: Tong He <hetong007@gmail.com>