fn diag(&self) -> Vec<f64> {
        let mut container = vec![0f64; self.row];
        let r = self.row;
        let c = self.col;
        assert_eq!(r, c);
        let c2 = c + 1;
        for i in 0..r {
            container[i] = self.data[i * c2];
        }
        container
    }
Extract diagonal components

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

fn main() {
let a = matrix!(1;4;1, 2, 2, Row);
assert_eq!(a.diag(), c!(1,4));
}
```
src/structure/matrix.rs