Specify that areacella is needed for area statistics in the Python example recipe

[bouweandela]

Description

The Python example recipe includes an area statistics preprocessor function. It was not specified that this should use areacella (and not areacello). If a new user without any data available tries to run the recipe with the download option, it will download areacello, which might be confusing. This pull request fixes the problem by specifying which cell measure variable is needed.

See also discussion in ESMValGroup/ESMValCore#1265

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[bouweandela]

@esmvalbot Please run examples/recipe_python.yml

[esmvalbot]

Since @bouweandela asked, ESMValBot will run recipe examples/recipe_python.yml as soon as possible, output will be generated here

[esmvalbot]

ESMValBot is happy to report recipe examples/recipe_python.yml ran OK, output has been generated here