Split regional historical recipes into model and reference runs

[bouweandela]

Description

Split regional historical recipes into model and reference runs. This avoids rerunning relatively expensive computations to compute the reference for each model. The downside is that the plots created by ESMValTool now will only contain the model or reference, so it is up to the REF portal to create a series figure where both are visible.

Checklist

Please confirm that this pull request has done the following:

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

TODO

• Update regression tests so they can run multiple versions of the same diagnostic, one for each data requirement
• Update the regional trends diagnostic

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...valtool/diagnostics/regional_historical_changes.py	91.30%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
...valtool/diagnostics/regional_historical_changes.py	96.36% <91.30%> (-1.58%)	⬇️

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