fn as_mut_slice(&mut self) -> &mut [f64] {
        &mut self.data[..]
    }
Mutable slice of `self.data`

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

let mut a = matrix(vec![1,2,3,4], 2, 2, Col);
let mut b = a.as_mut_slice();
b[0] = 5f64;
assert_eq!(b, &[5f64, 2f64, 3f64, 4f64]);
assert_eq!(a, matrix(vec![5,2,3,4], 2, 2, Col));
```
src/structure/matrix.rs