code: def get_word_saliency(topic_word_distrib, doc_topic_distrib, doc_lengths):
      
      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)
docs: 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"