Returns the normalization constant:$$
\frac{
\prod_{i=1}^{k}\Gamma\left(\alpha_i\right)
}{
\Gamma\left(\sum_{i=1}^{k}\alpha_i\right)
}
$$
for the Dirichlet and
$$
\frac{\prod_{i=1}^{k}\Gamma\left(a_i\right)\Gamma\left(b_i\right)
}{
\prod_{i=1}^{k}\Gamma\left(a_i+b_i\right)
}
$$
for the generalized Dirichlet.