fn subs_mat(&mut self, start: (usize, usize), end: (usize, usize), m: &ComplexMatrix) {
        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 complex matrix to specific position

# Description
Substitute below elements of complex 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::*;
use peroxide::complex::matrix::*;

fn main() {
let mut a = ml_cmatrix("1.0+1.0i 2.0+2.0i 3.0+3.0i;
4.0+4.0i 5.0+5.0i 6.0+6.0i;
7.0+7.0i 8.0+8.0i 9.0+9.0i");
let b = cmatrix(vec![C64::new(1f64, 1f64),
C64::new(2f64, 2f64),
C64::new(3f64, 3f64),
C64::new(4f64, 4f64)],
2, 2, Row);
let c = ml_cmatrix("1.0+1.0i 2.0+2.0i 3.0+3.0i;
4.0+4.0i 1.0+1.0i 2.0+2.0i;
7.0+7.0i 3.0+3.0i 4.0+4.0i");
a.subs_mat((1,1), (2,2), &b);
assert_eq!(a, c);
}
```
src/complex/matrix.rs