Add sea ice sensitivity diagnostic

[bouweandela]

Description

Closes #200

Sample data available in Climate-REF/ref-sample-data#45

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

❌ Patch coverage is 98.41270% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...imate-ref-core/src/climate_ref_core/constraints.py	90.90%	0 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
...src/climate_ref_esmvaltool/diagnostics/__init__.py	100.00% <100.00%> (ø)
...ool/src/climate_ref_esmvaltool/diagnostics/base.py	100.00% <100.00%> (ø)
..._ref_esmvaltool/diagnostics/sea_ice_sensitivity.py	100.00% <100.00%> (ø)
...ef-esmvaltool/src/climate_ref_esmvaltool/recipe.py	92.85% <100.00%> (-0.13%)	⬇️
...imate-ref-core/src/climate_ref_core/constraints.py	94.21% <90.90%> (-0.62%)	⬇️

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[lewisjared]

Merging as the new sample data will be available in a few hours and the integration tests are already broken :D