Add a `sample_dirichlet_multinomial` function to `stats_utils`

[bmmalone]

Here is a basic example:

def sample_dirichlet_multinomial(dirichlet_alphas:np.ndarray, num_samples:int) -> np.ndarray:
    pvals = np.random.dirichlet(dirichlet_alphas)
    sampled_counts = np.random.multinomial(n=num_samples, pvals=pvals)
    return sampled_counts