Function math::stats::percentile_by

pub fn percentile_by<T, F>(
    numbers: Vec<T>, 
    percentile_ratio: f64, 
    compare: F
) -> Result<T, String> where
    T: Clone,
    F: FnMut(&T, &T) -> Ordering, 

percentile_ratio is percentile / 100, e.g. the 90-th percentile corresponds to a percentile_ratio of 0.9.