Alternative xarray approach to computing convservation time series

[xylar]

This alternative approach makes sure the contents are unique (and sorted) in the xtime variable.

To reduce redundancy between the new approach and the old ncrcat approach (retained for comparison and in case the xarray approach is not performant), a new helper function for determining if we want to append to the existing datasets added.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran just the conservation analysis on 20250327.v3.SORRME3r3.piControl.alfred2.chrysalis and it looks reasonable:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xylar/analysis_testing/chrysalis/fix-conservation-redundant-time/20250327.v3.SORRME3r3.piControl.alfred2.chrysalis/

I am also running the test suite and the conservation analysis looks the same as before in tests that have completed so far there:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xylar/analysis_testing/chrysalis/fix-conservation-redundant-time/

As an example, here is a plot of salt conservation prior to this PR:

and with this PR:

[xylar]

@cbegeman and @darincomeau, if you could have a look, that would be great. If we're good with these fixes, I can merge this and update the analysis environment for BlueTip to include this fix.

[cbegeman]

Approving on the basis of code inspection and @xylar's testing. Thanks for fixing this!

[xylar]

Thanks @cbegeman!

[darincomeau]

Thanks @xylar for the quick fix! You're testing looks good to me. I can approve now (it'll be easier to use this in zppy when it's in the new bluetip environment), or I can run the full analysis from here on that alfred2 run.

[xylar]

I think you can just approve. It's not too hard for me to redo thing if something goes wrong later with the updated BlueTip environment.

[darincomeau]

Approved based on developer testing. Thanks again Xylar!

[xylar]

Thanks @darincomeau. I'll try to put a new environment in place soon.