Creates an S3 object representing a beta distribution with shape
parameters shape1 and shape2. The PDF on (0,1) is
f(x)=xa−1(1−x)b−1B(a,b)
shape1, b = shape2, and B(a,b)
is the beta function.
beta_dist(shape1, shape2)A beta_dist object with classes
c("beta_dist", "univariate_dist", "continuous_dist", "dist").