Implementing simplified Betts Miller convection

[AndrewILWilliams]

Describe your changes

WORK IN PROGRESS, nothing runs yet (thanks Claude).

Blows up after ≈ 10 hrs, generating 200% RH after a few hours in lower levels

Issue ticket number and link

Linked to #314

Checklist before requesting a review

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[AndrewILWilliams]

It does raise a question about API though, @duncanwp . I've just overwritten SPEEDY's convection scheme for now, but instead maybe it's better to have a few options for convection scheme (related to #206 ).

Anyways, for now the thing doesn't even work...baby steps