[DEV] Add memory storage to ChemGraph

[tdpham2]

Allow users to continue previous conversation via checkpoint.

See LangGraph documentation: https://docs.langchain.com/oss/python/langgraph/persistence, under Checkpointer libraries.

[tdpham2]

See https://docs.langchain.com/oss/python/langgraph/add-memory#add-long-term-memory

[tdpham2]

See PR: #101