pub fn cov(v1: &Vec<f64>, v2: &Vec<f64>) -> f64 {
    let mut ss = 0f64;
    let mut sx = 0f64;
    let mut sy = 0f64;
    let mut l = 0f64;

    for (x, y) in v1.into_iter().zip(v2) {
        ss += x * y;
        sx += *x;
        sy += *y;
        l += 1f64;
    }
    assert_ne!(l, 1f64);
    (ss / l - (sx * sy) / (l.powf(2f64))) * l / (l - 1f64)
}
Covariance (to Value)

# Examples
```
#[macro_use]
extern crate peroxide;
use peroxide::fuga::*;

fn main() {
let v1 = c!(1,2,3);
let v2 = c!(3,2,1);
assert!(nearly_eq(cov(&v1, &v2), -1f64));
}
```
src/statistics/stat.rs