fn eigsrt(d: &mut Vec<f64>, v: &mut Matrix) {
    let mut k: usize;
    let n = d.len();
    for i in 0..n - 1 {
        k = i;
        let mut p = d[k];
        for j in i..n {
            if d[j] >= p {
                k = j;
                p = d[k];
            }
        }
        if k != i {
            d[k] = d[i];
            d[i] = p;
            for j in 0..n {
                p = v[(j, i)];
                v[(j, i)] = v[(j, k)];
                v[(j, k)] = p;
            }
        }
    }
}
Given eigenvalue & eigenvector, sorts thod eigenvalues into descending order

* Reference : Press, William H., and William T. Vetterling. *Numerical Recipes.* Cambridge: Cambridge Univ. Press, 2007.
src/numerical/eigen.rs