fn subs_mat(&mut self, start: (usize, usize), end: (usize, usize), m: &Matrix) {
        let row = end.0 - start.0 + 1;
        let col = end.1 - start.1 + 1;
        for i in 0..row {
            for j in 0..col {
                self[(start.0 + i, start.1 + j)] = m[(i, j)];
            }
        }
    }
Substitute matrix to specific position

# Description
Substitute below elements of matrix

$$
\begin{pmatrix}
\\ddots & & & & \\\\
& start & \\cdots & end.1 & \\\\
& \\vdots & \\ddots & \\vdots & \\\\
& end.0 & \\cdots & end & \\\\
& & & & \\ddots
\end{pmatrix}
$$

# Examples
```
extern crate peroxide;
use peroxide::fuga::*;

fn main() {
let mut a = ml_matrix("1 2 3;4 5 6;7 8 9");
let b = ml_matrix("1 2;3 4");
let c = ml_matrix("1 2 3;4 1 2;7 3 4");
a.subs_mat((1,1), (2,2), &b);
assert_eq!(a, c);
}
```
src/structure/matrix.rs