restructure component models

[juliasloan25]

Purpose

separate required vs model-specific (internal) functions and objects in component model files

follows off of #644

Content

• distinguish between coupler-required and model-specific functions in component model files
• merge bucket_init.jl and bucket_utils.jl
• rename component model files to <model>.jl
• alphabetize get_field and update_field! methods for easier comparison/readability

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• I have read and checked the items on the review checklist.

[LenkaNovak]

It looks like the rebase removed some of the changes in the previous PR, e.g. Spaces -> ClimaCore.Spaces , and perhaps some of the imports. It may be good to look over the changes here just to double check. It would be good to check that there are no behavior changes both for slabplanet and AMIP before merging. Otherwise LGTM, thank you for doing this! Approving, but I'm happy to take a look again if needed.

[LenkaNovak]

I noticed recently that sometimes the docstrings of extensions can sometimes cause the extensions to be ignored. It doesn't seem to be problematic here, but we should probably decide on a convention (Clima wide) as to how to deal with this. (not as part of this PR)

[juliasloan25]

good point!