adopt kd-tree for data structure of node list

[medalotte]

Currently, nodes are stored to std::vector, and the NN and NBHD search are implemented by linear search.
In high dimensional space, to solve effectively above problems, kd-tree is suitable data structure.
https://ieeexplore.ieee.org/document/4084578

[medalotte]

I merged feature of simple kd-tree which rebuilds the tree when a depth of the tree exceeds certain ratio * log(size of tree).
#2

[medalotte]

It is probably better to implement a kd tree that is always in equilibrium.