[develop] Replace manage_externals with Git submodules

[MichaelLueken]

DESCRIPTION OF CHANGES:

As voted on in the first SRW App CM meeting in 2024, this PR replaces the current manage_externals with Git submodules. This PR will remain in draft form until we are ready to proceed with this update.

Type of change

• [X] New feature (non-breaking change which adds functionality) - Adding submodules
• [X] Breaking change (fix or feature that would cause existing functionality to not work as expected) - Removing current manage_externals capability
• [X] This change requires a documentation update - Documentation and images using manage_externals will need to be removed/updated with submodules

TESTS CONDUCTED:

• [X] hera.intel
• [ ] orion.intel
• [ ] hercules.intel
• [ ] cheyenne.intel
• [ ] cheyenne.gnu
• [ ] derecho.intel
• [ ] gaea.intel
• [ ] gaeac5.intel
• [ ] jet.intel
• [ ] Jenkins
• [X] fundamental test suite
• [ ] comprehensive tests (specify which if a subset was used)

DEPENDENCIES:

None

DOCUMENTATION:

Documentation will be updated in this PR before it is opened.

ISSUE:

CHECKLIST

• [X] My code follows the style guidelines in the Contributor's Guide
• [X] I have performed a self-review of my own code using the Code Reviewer's Guide
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] My changes need updates to the documentation. I have made corresponding changes to the documentation
• [ ] My changes do not require updates to the documentation (explain).
• [X] My changes generate no new warnings
• [X] New and existing tests pass with my changes
• [ ] Any dependent changes have been merged and published

[MichaelLueken]

To clone the repository, you can use either:

git clone --recursive -b feature/git_submodules git@github.com:MichaelLueken/ufs-srweather-app.git

or, if you didn't use the --recursive option:

git clone -b feature/git_submodules git@github.com:MichaelLueken/ufs-srweather-app.git cd ufs-srweather-app git submodule update --init --recursive