Extend preprocessor multi_model_statistics to handle data with "altitude" coordinate

[axel-lauer]

This PR extends the preprocessor multi_model_statistics to allow handling of data with a vertical corrdinate "altitude".

• Closes Preprocessor multi_model_statistics cannot handle altitude coordinate #1009

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[schlunma]

One small suggestion attached. Tested successfully with real data 👍

This is only a temporary solution, but since the multi_model_statistics preprocessor will be lazy soon and hopefully support arbitrary input dimensions (#968), I guess we can merge this small workaround as is.

[valeriupredoi]

nice one @axel-lauer - please add relevant review labels next time, gonna add the passed tech review now - nevermind, we don't have them in Core 👍