Add an efficient implementation of Kendall's Tau-b

This is supposedly the algorithm proposed by Knight (reference
in the source code comments), but really, this developer used the
description on Wikipedia (it is really strange how few scientists
understand what a big obstacle the current publishing system is to
proper scientific work).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use the fast Kendall Tau-b algorithm instead of the naive one

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Kendall-Tau: add an exhaustive test

It is always good to verify that a robust but slow algorithm is replaced
by a fast *and accurate* algorithm.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>