Function jacobi_preconditioned_cg 

pub fn jacobi_preconditioned_cg(
    a: &CsrMatrix,
    b: &[f64],
    max_iter: usize,
    tol: f64,
) -> (Vec<f64>, usize)

Conjugate Gradient with Jacobi (diagonal) preconditioner for A x = b (SPD).

Returns (solution, iterations_used).