Remaining tasks

[mdpiper]

In case I don't get back to this soon, here's a list of remaining tasks.

• Add a pseudo-diffusivity to the model; make it a parameter (Add a pseudo diffusivity to the model #3)
• Describe boundary conditions in README
• Make a notebook (or two) to run the model through its BMI (Add BMI example notebook #4)
• Add a README for the examples directory
• Add a changelog
• Tag a release
• Write a blurb for Bsky, the Forum, and the newsletter