New optional argument for selecting log-likelihood

[bab2min]

Suggested by #147.

result_corpus, ll_per_doc = mdl.infer(corpus)

Currently, ll_per_doc are actually sums of ll for doc-topic and ll for topic-term in above code. But depending on the situation, it may be convenient to return a different ll value. So suggest following a new argument:

result_corpus, ll_per_doc = mdl.infer(corpus, ll=ll_type)

A valid ll_type is one of followings:

• doc+topic: sum of ll for doc-topic and ll for topic-term
• doc: only ll for doc-topic
• topic: only ll for topic-term
• doc+topic/w: same as doc+topic, but per-word
• doc/w: same as doc, but per-word
• topic/w: same as topic, but per-word