Make methods consistent

[tdhopper]

The term_relevance_by_topic method of the state class returns a list (over the topics) of lists. The internal lists contain tuples where the first element is the word and the second is the the term relevance score.

The word_distribution_by_topic method returns a list (over the topics) of dicts. The dicts are mappings from vocabulary words to probabilities of the word being generated by the given topic.

We should make the data structures of these more similar as well as make the names more consistent. ("Word" and "term" are being used interchangeably.)