Add mypy and fix issues

[MarcSkovMadsen]

When I look at the lumen code in VS Code I see errors marked all over the place by mypy. Please please please use mypy to help create a robust code base with a minimum of issues. Thanks.

[MarcSkovMadsen]

When running mypy on a repository that is using lumen I see

src/mt_ai_assistant/data_assistant.py:7: error: Skipping analyzing "lumen.ai": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/mt_ai_assistant/data_assistant.py:7: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
src/mt_ai_assistant/data_assistant.py:7: error: Skipping analyzing "lumen": module is installed, but missing library stubs or py.typed marker

Please remember to add py.typed marker file.