aggregate_doc_topics has un-intuitive UI

[JaimieMurdock]

Almost everytime I use aggregate_doc_topics I have to perform the following transformation to get an array of the topic distribution:

# v.aggregate_doc_topics returns an ndarray instead of just values.
theta = v.aggregate_doc_topics(all_ids)
theta = theta[np.argsort(theta['i'])]
theta = np.array(theta['value'])

I should add an aggregate_doc_topic_matrix to return this properly formatted list, or clean up our DataTable implementation. Since the second seems super hard, I'm going to go with the first.