Fix extreme events and extreme index ancestors

[katjaweigel]

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Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• Give this pull request a descriptive title that can be used as a one line summary in a changelog
• Make sure your code is composed of functions of no more than 50 lines and uses meaningful names for variables
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• Please use yamllint to check that your YAML files do not contain mistakes
• (Only if really necessary) Add any additional dependencies needed for the diagnostic script to setup.py, esmvaltool/install/R/r_requirements.txt or esmvaltool/install/Julia/Project.toml (depending on the language of your script) and also to package/meta.yaml for conda dependencies (includes Python and others, but not R/Julia)
• If new dependencies are introduced, check that the license is compatible with Apache2.0

New recipe/diagnostic

• Add documentation for the recipe to the doc/sphinx/source/recipes folder and add a new entry to index.rst
• Add provenance information

Modified recipe/diagnostic

• Update documentation for the recipe to the doc/sphinx/source/recipes folder
• Update provenance information if needed
• Assign the author(s) of the affected recipe(s) as reviewer(s)

New data reformatting script

• Test the CMORized data using recipes/example/recipe_check_obs.yml, to make sure the CMOR checks pass without errors
• Add the new dataset to the table in the documentation
• Tag @mattiarighi in this pull request, so that the new dataset can be added to the OBS data pool at DKRZ and synchronized with CEDA-Jasmin

Modified data reformatting script

• Test the CMORized data using recipes/example/recipe_check_obs.yml, to make sure the CMOR checks pass without errors
• Tag @mattiarighi in this pull request, so that the updated dataset can be added to the OBS data pool at DKRZ and synchronized with CEDA-Jasmin

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

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Closes #1718

[katjaweigel]

For me it runs like that, please check!
It looks like ancestors for provenance can either be set as list or as vector of characters in R, but not as the combination of both to be a non nested list in the (phython) Core.

[mattiarighi]

Thank you @katjaweigel

[mattiarighi]

To be cherry-picked...

[bouweandela]

Done