Fit various smoothing spline models. Includes an ssr() function for smoothing
spline regression, an nnr() function for nonparametric nonlinear regression, an snr()
function for semiparametric nonlinear regression, an slm() function for semiparametric
linear mixed-effects models, and an snm() function for semiparametric nonlinear
mixed-effects models. See Wang (2011)