Kennard-stone

**Full Changelog**: https://github.com/yu9824/kennard_stone/compare/v2.1.2...v2.1.3

- Fix bug when `metric="nan_euclidean"`.

- Optimize algorithm
- Deal with Large number of data.
- parallel calculation when calculating distance (Add `n_jobs` argument)
- recursion number settings
- Add other than "euclidean" calculation methods (Add `metric` argument)

Fix bug with Python 3.7.

**This version has a bug with Python 3.7. Use with the other Python versions is not a problem, but I recommend using newer versions.**

- Define Extended Kennard-Stone algorithm (multi-class) i.e. Improve `KFold` algorithm.
- Delete alternate argument in `KFold`.
- Delete requirements of pandas.