Function f_statistic

fn f_statistic(rss_r: f64, rss_u: f64, q: usize, n: usize, p_u: usize) -> f64

Compute F-statistic from restricted and unrestricted RSS.

F = ((RSS_r - RSS_u) / q) / (RSS_u / (n - p_u)) where q = extra parameters in unrestricted model, p_u = total params.