* Add `data.dataset.SequentialDataset` to support new data formats:
  * "SIT": Session, Item, Timestamp
  * "SITJson": Session, Item, Timestamp, Json
  * "USIT": User, Session, Item, Timestamp
  * "USITJson": User, Session, Item, Timestamp, Json
* Add `data.reader` parser for listed formats
* Add `eval_method.NextItemEvaluation` for next-item evaluation
* Add `models.SPop` baseline model: session-based popular
* Add `gowalla` check-ins and `yoochoose` datasets
* Add unit tests

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Co-authored-by: lthoang <trhoanglee@gmail.com>
Co-authored-by: tqtg <tuantq.vnu@gmail.com>

* Fix eval and add batch basket items iteration

* refactor code

* Fix scoring function

* refactor code

* Add unittest for NextBasketEvaluation

* Add unittest for BasketDataset

* Add test NextBasketRecommender

* Add test case reading basket data

* refactor code

* Add history basket ids for accessing extra data in scoring function

* Add option to scoring function based on quantity provided in extra_data

* refactor code

* reuse user_iter() in ub_iter()

* reuse ub_iter() in ubi_iter()

* consider using num_user_batches() instead of overwriting existing num_batches()

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Co-authored-by: tqtg <tuantq.vnu@gmail.com>