log binomial coefficient with extended gamma factorials. n and k_agg may be
    int or array - if both array, must be the same length.

def _ln_choose(n, k_agg):
    '''
    log binomial coefficient with extended gamma factorials. n and k_agg may be
    int or array - if both array, must be the same length.

    '''
    gammaln = special.gammaln
    return gammaln(n + 1) - (gammaln(k_agg + 1) + gammaln(n - k_agg + 1))