@rnd_gamma(α:float)
returns a random generator that produces floating-point values according to a gamma distribution P:
P(x | α) = x^(α-1) / Γ(α), x ≥ 0.
See @rnd_bernoulli for a description of random generators.
See also Random Generators @rand @rand_int @random @rnd_bernoulli @rnd_binomial @rnd_exponential @rnd_gamma @rnd_geometric @rnd_normal @rnd_uniform_float @rnd_uniform_int