major revision. 1 train_predict_wordhood: now wordhood() does not only consider the num of actual words in a candidate's most similar items, also consider the average score of these words.
2.trigram_corpus_gen:  now one func for gen trigram corpus from train_corpus and one for that from test_corpus;
3. train.py and predict.py manily adjust according to 2 and for the purposes of experimentation.