Add kendall tau distance (not as a submodule)

[unna97]

• tests are still a separate file: shift it docstring as examples

[lvwerra]

Looks much better :) Will review in a bit!

[unna97]

• Add a more efficient algorithm to calculate Kendall tau using bubble sort algorithm: current uses brute force O(n^2), it can be reduced to O(n*log(n))
• Add weighted Kendall tau distance: https://link.springer.com/article/10.1007/s00026-020-00519-y

[unna97]

@lvwerra Can I get a timeline on when this pull request be reviewed?