CG_smooth               High-dimensional spline smoothing using a
                        matrix-free CG-method.
MGCG_smooth             High-dimensional spline smoothing using a
                        matrix-free multigrid preconditioned CG-method.
PCG_smooth              High-dimensional spline smoothing using a
                        matrix-free PCG-method.
estimate_trace          Trace estimation of the hat matrix.
generate_test_data      Generate multi-dimensional test data for spline
                        smoothing.
predict_smooth          Predictions from model
