Improve RetrieveChat (#6)

* Upsert in batch

* Improve update context, support customized answer prefix

* Update tests

* Update intermediate answer

* Fix duplicate intermediate answer, add example 6 to notebook

* Add notebook results

* Works better without intermediate answers in the context

* Bump version to 0.1.2

* Remove commented code and add descriptions to _generate_retrieve_user_reply

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Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>