Modify FV3 code for unit testing

[dpsarmie]

Description

This PR is now a combination of other PRs. This includes the following:

• The original #966 PR that made modifications to the FV3 code to create a few submodules to allow for easier unit testing.
• #967 which added unit test files to the ./tests/ directory.
• #975 which fixes the CI/CD workflow for the FV3 repo.

These three PRs are working towards the similar goal, so they were combined so it can be more easily and quickly added to the UFS repo.

There are additional tests in my fix/cicd branch for subroutines in the moving nest folder, but I believe this requires more work to the CI/CD workflow. There are other tests that will also require some dummy data for unit testing.

This PR will be the first batch of tests that can be added to FV3 with other tests added in the near future.

Issue(s) addressed

Link the issues to be closed with this PR, whether in this repository, or in another repository.
(Remember, issues should always be created before starting work on a PR branch!)

• fixes Adding unit tests to FV3 #965
• fixes Github actions are failing in 'build_fv3atm' steps #959
• closes Add unit test files #967 (PR changes were merged into this PR)
• closes Fix CI/CD workflow #975 (PR changes were merges into this PR)

Testing

How were these changes tested? Using the UFS RT suite
What compilers / HPCs was it tested with? Tests were done on Hera
Are the changes covered by regression tests? Yes
Have the ufs-weather-model regression test been run? On what platform? Yes, Hera

• Will the code updates change regression test baseline? No
• Please commit the regression test log files in your ufs-weather-model branch

Dependencies

Do PRs in upstream repositories need to be merged first? No

[dpsarmie]

Just combined changes from PR #967 and #975 into this PR. I changed the description to reflect these changes and the other PRs were closed with a similar comment.

This PR now fixes the CI/CD workflow and adds the first batch of unit tests for FV3.

[AlexanderRichert-NOAA]

This looks good to me, and it's great to see a batch of new unit tests. The only suggestion I'd offer would be to add error messages before the 'stop' calls in the unit tests if you thought it would facilitate investigating failed tests.

[dpsarmie]

Went ahead and added some error messages to the tests. I'll start getting the PR up on the UFS side.