Calculate word distinctiveness according to Chuang et al. 2012.
    distinctiveness(w) = KL(P(T|w), P(T)) = sum_T(P(T|w) log(P(T|w)/P(T)))
    with P(T) .. marginal topic distribution
         P(T|w) .. prob. of a topic given a word

    J. Chuang, C. Manning, J. Heer 2012: "Termite: Visualization Techniques for Assessing Textual Topic Models"

def get_word_distinctiveness(topic_word_distrib, p_t):
    """
    Calculate word distinctiveness according to Chuang et al. 2012.
    distinctiveness(w) = KL(P(T|w), P(T)) = sum_T(P(T|w) log(P(T|w)/P(T)))
    with P(T) .. marginal topic distribution
         P(T|w) .. prob. of a topic given a word

    J. Chuang, C. Manning, J. Heer 2012: "Termite: Visualization Techniques for Assessing Textual Topic Models"
    """
    topic_given_w = topic_word_distrib / topic_word_distrib.sum(axis=0)
    return (topic_given_w * np.log(topic_given_w.T / p_t).T).sum(axis=0)