Extends the class {@link ContinuousDistribution} forthe
gamma distribution with shape parameter
α > 0 and scale parameter
λ > 0. The density is
f (x) = λαxα-1e-λx/Γ(α), for x > 0,
where
Γ is the gamma function, defined by
Γ(α) = ∫0∞xα-1e-xdx.
In particular,
Γ(n) = (n - 1)! when
n is a positive integer.