Notebook/hierarchy flow (#482)

* Notebook showing how to use select speaker to control conversation flow.

* pytest associated with notebook.

* Added llm_config to assistant and user proxy agent, and clarified why we set use_cache to false, as requested in the review.

* Added a @pytest.mark.skipif decorator like other tests to run it only in one py version, 3.10

* Fixed config warning.

* Removd llm_config to UserProxyAgent

* Fixed minor typos.

* Reran outputs

* Remopved llm_config from user_proxy_agent

* Colab Badge link updated.

* pre-commit formatting changes.

* Fixed base_url

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Co-authored-by: Chi Wang <[email protected]>