Fixed derivation of co2s

[schlunma]

This PR fixes an error in the derivation script of co2s. In order to represent the true CO2 mole fraction at surface, it is necessary to interpolate the data on the surface air pressure. A recipe to test these changes can be found in #587.

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)
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• 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
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• 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.

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Closes #600.

[bouweandela]

Code looks good to me, but this would need scientific review too. Could you please always create issues for bugs, because that way people can find out that there was a bug (and it is now solved). Not everyone might notice a merged pull request.

[bettina-gier]

Looks good to me now!
Tested it with both a CMIP5 and CMIP6 model, ran without issues and result looks plausible.