Calculate word saliency according to Chuang et al. 2012.
    saliency(w) = p(w) * distinctiveness(w)

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

def get_word_saliency(topic_word_distrib, doc_topic_distrib, doc_lengths):
    """
    Calculate word saliency according to Chuang et al. 2012.
    saliency(w) = p(w) * distinctiveness(w)

    J. Chuang, C. Manning, J. Heer 2012: "Termite: Visualization Techniques for Assessing Textual Topic Models"
    """
    p_t = get_marginal_topic_distrib(doc_topic_distrib, doc_lengths)
    p_w = get_marginal_word_distrib(topic_word_distrib, p_t)

    return p_w * get_word_distinctiveness(topic_word_distrib, p_t)