Function gpu_pcg_solver 

pub fn gpu_pcg_solver(
    mat: &SparseMatrixGpu,
    b: &[f64],
    precond: &[f64],
    max_iter: usize,
    tol: f64,
) -> (Vec<f64>, usize, f64)

Preconditioned conjugate gradient solver.

precond should be a vector of reciprocal diagonal entries (from gpu_jacobi_preconditioner). Returns (x, iterations, residual_norm).