Take advantage of xarray's decode_times for Omega's time variable conversion

[cbegeman]

The most substantive change here is adding a new way to convert Omega's time variable to daysSinceStartOfSim when xarray Datasets are loaded with decode_times=True. The rest is a bit of clean-up and using this method in the seamount test case just to make sure that the clean-up didn't break MPAS-O xtime conversion.

Checklist

• [N/A] User's Guide has been updated
• [N/A] Developer's Guide has been updated
• [N/A] API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• [N/A] Documentation has been built locally and changes look as expected
• Testing comment in the PR documents testing used to verify the changes
• [N/A] New tests have been added to a test suite

[cbegeman]

Testing

The full omega_pr test suite ran successfully. Some of the baseline comparisons failed because I used an older test instead of re-running with main.

### Polaris `omega_pr` suite
- Baseline workdir: `/pscratch/sd/c/cbegeman/polaris-scratch/ztilde-omega-develop-20260427`
- Baseline build: `/pscratch/sd/c/cbegeman/polaris-scratch/ztilde-omega-develop-20260427/build`
- PR build: `/pscratch/sd/c/cbegeman/polaris-scratch/ztilde-omega-develop-20260427/build`
- PR workdir: `/pscratch/sd/c/cbegeman/polaris-scratch/enhancetime-omega-develop-20260429`
- Machine: `pm-cpu`
- Compiler: `gnu`
- Build type: `Release`
- Log: not found
- Result:
  - Diffs (2 of 10):
    - `ocean/column/horiz_press_grad/salinity_gradient`
    - `ocean/planar/barotropic_gyre/munk/free-slip`

[cbegeman]

@xylar Just a reminder to review when you have a chance

[xylar]

Yes, it's on my list. Thanks for the reminder.

[xylar]

I have tests in the Frontier queue. I'll report back as soon as they're done. The code changes look good to me! It makes sense to take advantage of the Omega time coordinate where we can.

[xylar]

Looks great!

Polaris omega_pr suite

• Baseline workdir: /lustre/orion/cli115/scratch/xylar/polaris_1.0/frontier/test_20260504/omega-pr-baseline
• Baseline build: /lustre/orion/cli115/scratch/xylar/polaris_1.0/frontier/test_20260504/omega-pr-baseline/build
• PR build: /lustre/orion/cli115/scratch/xylar/polaris_1.0/frontier/test_20260504/omega-pr-baseline/build
• PR workdir: /lustre/orion/cli115/scratch/xylar/polaris_1.0/frontier/test_20260504/omega-pr-xarray-time
• Machine: frontier
• Partition: batch
• Compiler: craygnu
• Build type: Release
• Log: /lustre/orion/cli115/scratch/xylar/polaris_1.0/frontier/test_20260504/omega-pr-xarray-time/polaris_omega_pr.o4527928
• Result: All tests passed

[cbegeman]

@xylar Thanks for the review and testing!