Rename and modify Model_Agnostic_Analysis.ipynb

[adele-morrison]

Rename this something like Cfxarray_and_Pint.ipynb.

Turn this into an advanced tutorial to show users how they could use cf-xarray and pint if they want in their own code.

Also add an explanation of why we don't use cf-xarray in the rest of the recipes, because we want them to be pedagogical and for people to learn about model grids/coordinates.