Function pcg_solve 

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

Preconditioned Conjugate Gradient solver.

precond[i] = 1 / a_ii (diagonal preconditioner). Returns (x, n_iter, residual_norm).