Factor out surface land fluxes

[duncanwp]

Describe your changes

Factor out the land surface flux code into a separate function and add tests specifically for it. They all pass (which is annoying because we're trying to track down an issue with precipitation over land)

Issue ticket number and link

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

[duncanwp]

We can keep this or not, up to you