fn to_vec(&self) -> Vec<Vec<f64>> {
        let mut result = vec![vec![0f64; self.col]; self.row];
        for i in 0..self.row {
            result[i] = self.row(i);
        }
        result
    }
Matrix to `Vec<Vec<f64>>`

To send `Matrix` to `inline-python`
src/structure/matrix.rs