Function cg_solve 

pub fn cg_solve(
    a: &CsrMatrix,
    b: &[f64],
    x0: &[f64],
    tol: f64,
    max_iter: usize,
) -> (Vec<f64>, u32, f64)

Conjugate Gradient solver for symmetric positive-definite systems A x = b.

Returns (x, n_iter, residual_norm).