implement Spearman rho

[UnixJunkie]

can be used to test if a model rank-orders things properly

[UnixJunkie]

https://en.wikipedia.org/wiki/Spearman's_rank_correlation_coefficient

[UnixJunkie]

compare pairs (x, i) (y, j) instead of x and y directly; use i to break ties: the comparator compares x and y first, then i and j if there is a tie

[UnixJunkie]

Spearman is Pearson of ranks; so implement Pearson first