Frequentist fitting utilities and notebook

[dr-david]

Added a jupyter notebook used to fit the model

[pawel-czyz]

Overall the code looked pretty good to me! I fixed some minor issues with packaging and refactored the notebooks into Quarto, so that it's easy to run in the interactive mode in VS Code.

However, the frequentist fitting notebook didn't work on my machine:

• There's Hessian needed, which doesn't work on my laptop.
• The shapes are non-trivial. Sometimes we have 7 variants and sometimes 8. I tried to systematize it, but I think I only broke it, so a careful look will be required.

I think I'll merge it now and then we can think how to fix these in future PRs...