Unable to multiply complex matrix with complex scalar from the left

Hi there,

It is currently not possible to multiply a complex matrix with a complex scalar from the left, as illustrated here:
```rust
use nalgebra::{Complex, Matrix2};

fn main() {
let mat = Matrix2::>::identity();
let c = Complex::::new(1.0, -1.0);

let product = mat * c; // This works
let product = c * mat; // This does not compile
}
```

I suspect this is because `Complex` might be an external type. Is there a way to fix this currently or is it not possible? It is not a huge issue since you can always swap the multiplication order, but it is annoying.