Added preprocessor mask_multimodel

[schlunma]

This PR adds a working multimodel masking preprocessor.

The following things need to be implemented before the draft status can be cleared:

• Add documentation (and remove the old, wrong parts)
• Add unit tests
• Discuss about the name of the preprocessor, I do not think that mask_multimodel is the optimal choice.
• Adapt provenance of multimodel output

I probably do not have time to finish this soon, so any contributions to this PR are welcome!

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)
• This pull request has a descriptive title that can be used in a changelog
• Add unit tests
• Public functions should have a numpy-style docstring so they appear properly in the API documentation. For all other functions a one line docstring is sufficient.
• If writing a new/modified preprocessor function, please update the documentation
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.
• Please use yamllint to check that your YAML files do not contain mistakes
• If you make backward incompatible changes to the recipe format, make a new pull request in the ESMValTool repository and add the link below

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 #487.

[bascrezee]

To me these changes look good. I also tested this preprocessor quite thoroughly by now, and it works as expected.

[valeriupredoi]

@schlunma you wanna switch this PR from Draft to RfR, mate? I'll haz a look right after you did that 👍

[schlunma]

I still need to add tests and documentation for this, will do that this week/next week. After that I will switch the status of this PR 👍

[valeriupredoi]

I still need to add tests and documentation for this, will do that this week/next week. After that I will switch the status of this PR +1

rock on, dude! 🤘

[schlunma]

This is ready for review now 👍

I added unit tests and documentation and also tested it on real data with the recipe mentioned in #487. All works as expected.

[valeriupredoi]

looks good! a couple minor points inline, but also - could you pls add an integration test inside test_recipe where you call the preprocessor and do some stuff with it? Cheers, dude 🍺

[valeriupredoi]

excellent stuff, cheers much @schlunma 🍺