First decile and last decile

[Odonno]

It could be interesting to provide firstDecile and lastDecile functions which are often used in statistics.

I already wrote something like that:

const decile = (array, decile) => percentile(array, decile / 10); // if percentile is between 0 and 1
const firstDecile = (array) => decile(array, 1);
const lastDecile = (array) => decile(array, 9);

Also, notice that I would have expect percentile parameter to be between 0 and 100, like explained in the wikipedia documentation: https://en.wikipedia.org/wiki/Percentile#The_nearest-rank_method