fix e3sm_diags year looping for land model vs model

[chengzhuzhang]

Summary

The original design assumption was that ref_years and years would have the same number
of year sets, creating 1:1 pairings:

• Test period 1 vs Reference period 1
• Test period 2 vs Reference period 2
• etc.

One of the common use cases include single reference : ref_years = "451-500" with multiple test periods. This fix handles this common us case while keep the original design.

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Thanks @chengzhuzhang. If you've tested this on some relevant data sets, I can approve to merge.

[chengzhuzhang]

Yes, I tested with a recent run on v3.HR. I think it is okay to merge.