Make ECS diagnostic compatible with CMIP7 data

[schlunma]

Description

Addresses #647.

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
core	93.11% <ø> (ø)
providers	91.87% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...tool/src/climate_ref_esmvaltool/diagnostics/ecs.py	100.00% <100.00%> (+1.92%)	⬆️
...ef-esmvaltool/src/climate_ref_esmvaltool/recipe.py	86.61% <100.00%> (ø)

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[schlunma]

2 questions:

• Is it ok to point the ESMValTool commit hash to a feature branch or should this point to a commit in main? In other words, does Add CMIP7-compatible REF version of recipe_ecs.yml ESMValGroup/ESMValTool#4465 need to be merged before this?
• Running the test cases with ref test-cases run --provider esmvaltool --diagnostic equilibrium-climate-sensitivity --fetch --force-regen --clean creates a packages/climate-ref-esmvaltool/tests/test-data/ in my repository which is not gitignored. Is this a deliberate choice?

[bouweandela]

Is it ok to point the ESMValTool commit hash to a feature branch or should this point to a commit in main?

Using a feature branch is fine for testing, but the commit should be in ESMValTool's main branch before this pull request can be merged.

[schlunma]

Will mark this as "ready for review" once corresponding ESMValTool PR has been merged.