Function lanczos 

pub fn lanczos(
    a: &CsrMatrix,
    b0: &[f64],
    k: usize,
) -> (Vec<f64>, Vec<f64>, Vec<Vec<f64>>)

Lanczos iteration: build a tridiagonal approximation of a symmetric matrix A.

Returns (alphas, betas, Q) where:

• alphas[k] is the k-th diagonal of the tridiagonal matrix,
• betas[k] is the k-th subdiagonal (length k steps),
• Q[k] is the k-th Lanczos vector (orthonormal).