Adds a new word to the WordList and assigns it an id.  If id is set,
        :param word: The word token
        :param id: (Optional) The token id can be set, but bypasses the automatic id assignment.  Only specify the id
          if you plan to specify it on all add calls.
        :return: None

def add(self, word, id=None):
        """
        Adds a new word to the WordList and assigns it an id.  If id is set,
        :param word: The word token
        :param id: (Optional) The token id can be set, but bypasses the automatic id assignment.  Only specify the id
          if you plan to specify it on all add calls.
        :return: None
        """
        word_id = id if id is not None else self.get_next_id()
        self[word] = word_id
        self.id2word[word_id] = word