NRL updates: New template neptune-dev-llvm, update met/metplus

[climbfuji]

Description

This PR bundles a few updates for NRL:

1. New template neptune-dev-llvm for - as the name suggests - the LLVM compilers. We use this instead of the stripped-down neptune-ops template so that basic Python packages like py-numpy are available. The goal is to gradually fix builds with LLVM until we can switch over to the full neptune-dev template and delete the neptune-dev-llvm template.
2. Update met to 12.1.1 and metplus to 6.1.0
3. Make Python dependencies py-cartopy and py-matplotlib explicit in neptune-python-env, and by extension also in jedi-neptune-env. These packages are already built because of the py-xarray dependency, but it's good to list them explicitly in case we remove the py-xarray dependency at some point.

Dependencies

None.

Issues addressed

Closes #1847
Closes an issue in the NRL Enterprise GitHub related to updating met and metplus

Applications affected

Applications using met/metplus.

Systems affected

NRL systems using LLVM (Atlantis, Bounty).

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped (provide explanation if necessary)
• New tests added: List and describe any new tests added to GitHub actions
  • ...
• Additional testing: Add information on any additional tests conducted
  • Manual testing on Bounty with NEPTUNE
  • Testing on Atlantis with NEPTUNE

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the documentation (spack-stack wiki) will be made when this PR is merged - this step is needed and will be completed