more precise sequence for query_product
add comments for clarification
finish recommender preprocess
add recommend method
add HottestRecommender
add group_id column to query table in preparation for k-fold splitting
add k-fold splitter
paramterize query_train table
forbid access to config from within class
add tester and evaluator
can run a k-fold test now
add related product cache
this bring a speed up of about 5%
show round statistics