use log when craeting db
implement test for future cost estimate
implement test for future_cost_estimate function using db
detele test/:test:
revise testcode using with
implement test code for ngram model
implement ngram_count and corresponded createdb
implement create_ngram_count_except_last_view
revise view
revise phrase_prob using log probability in init_val
implement language_model
modify v for language model
revise stackdecoder to use language model