Add max, min and std operators to multimodel

[jvegreg]

Added several operators to the multimodel, as requested by @mwjury

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 #563

[valeriupredoi]

cheers Javi @jvegasbsc - I was supposed to do this but it completely slipped my mind 🍺 It's worth pointing out and I think we need to add it to the documentation that these are local (min, max) and not global

[jvegreg]

If we add a comment about it, we should do it for every operator. All are local

[valeriupredoi]

yeah I think we should say that, lemme add it to the doc

[bouweandela]

Nice work! @mattiarighi Could you please test?

[mattiarighi]

I have some installation issues at the moment (see #535) and cannot test.

[bouweandela]

@mattiarighi Did you manage to solve your installation issues?

[mattiarighi]

Yes, sorry for being absent lately, I'm very busy with other projects.
I'll try to test this and the other PRs this week.

[mattiarighi]

Seems to work as expected, thanks for adding this feature!