pub fn beta(r: &mut Rng, a: f64, b: f64) -> f64
This function returns a random variate from the beta distribution. The distribution function is,
p(x) dx = {Gamma(a+b) over Gamma(a) Gamma(b)} x^{a-1} (1-x)^{b-1} dx
for 0 <= x <= 1.