fn cov(&self) -> Self {
        let c = self.col;

        let mut m: Self = matrix(vec![0f64; c * c], c, c, self.shape);

        for i in 0..c {
            let m1 = self.col(i);
            for j in 0..c {
                let m2 = self.col(j);
                m[(i, j)] = cov(&m1, &m2);
            }
        }
        m
    }
Covariance Matrix (Column based)

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

fn main() {
let m = matrix(c!(1,2,3,3,2,1), 3, 2, Col);
println!("{}", m.cov());

//         c[0]    c[1]
// r[0]  1.0000 -1.0000
// r[1] -1.0000  1.0000
}
```
src/statistics/stat.rs