Adds Zonal Diffusion

[HenryDane]

This pull request adds zonal advection-diffusion, zonal heat diffusion, and zonal moist diffusion. Additionally, this makes the adv-diff solver support periodic boundary conditions.

I've also added some unit tests to verify that the changes work.

Please let me know if you have any comments or feedback or questions.

[brian-rose]

Hi @HenryDane, this looks awesome! Thanks for the contribution, and especially for including tests with your PR! It may take me a bit of time to digest everything you've done here.

[HenryDane]

I committed the change you suggested -- thanks very much for pointing that out to me. It looks like all the tests are passing now.

[brian-rose]

I committed the change you suggested -- thanks very much for pointing that out to me. It looks like all the tests are passing now.

Great! I'll take a closer look at the code when I get a chance.

[HenryDane]

I've been adding commits here (f256da through 4a6ba5b) to fix an issue I noticed with this new version where if a model had both zonal and meridional adv-diff processes, the fluxes (diffusive_flux, total_flux, advective_flux, flux_convergence) for one adv-diff process could get over-written by others.

These new commits fix this problem by adding an optional diag_suffix so that when a process inherits from AdvectionDiffusion, it can declare a suffix for the aforementioned attributes, thereby fixing the issue.

With these changes, I believe that zonal advection-diffusion, zonal heat diffusion, and zonal moist diffusion should now work correctly both on their own and in the presence of other adv-diff processes.

[brian-rose]

@HenryDane can you merge in the latest changes from main branch into this PR? That will clear up the unrelated test failures.

[HenryDane]

Yes, of course. I just applied the merge and all the tests seem to be passing now.