'Return the IDs of all the responses -- these are the keys used for
the dicts returned by grade_answers and get_question_answers. (Though
get_question_answers may only return a subset of these.'

def get_answer_ids(self):

    answer_ids = []
    for response in self.responders.keys():
        results = self.responder_answers[response]
        answer_ids.append(results.keys())
    return answer_ids
