Fix weight_manager problems in recommender, NN, anomaly and clustering

Fix #61.

* Fix fv_converter so that it doesn't generate implicit weight_manager instance.  Drivers are responsible for creating weight_manger and registering it via `set_weight_manager`; otherwise fv_converter will not store document frequencies used for IDF calculations.
* Fix recommender, NN, anomaly and clustering to obey the rule above.