beta() defines a one-response distribution for continuous proportions
strictly inside (0, 1), with formulas for mean mu and scale sigma.
Details
The implemented contract is
logit(mu) = eta_mu, log(sigma) = eta_sigma, and internal precision
phi = 1 / sigma^2. Larger sigma therefore means more variation around
the mean, not more precision.
This helper masks base::beta() when drmTMB is attached. Use
base::beta() for the mathematical beta function.