Added weightless_mean, weightless_sum operators to volume_statistics preprocessor

[ledm]

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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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Closes {442} in part. This issue was raised to address the problems in sum in area_statistics, but these same issues appear i nthe volume_statistics preprocessor.

[valeriupredoi]

when you run python setup.py test you can find the bits that not tested in eg file:///home/valeriu/ESMValCore/test-reports/coverage_html/esmvalcore_preprocessor__volume_py.html - have a look in yours and see there are a couple statements that are not tested, it'd be great if you could write a test or two on those 🍺

[bouweandela]

Wouldn't it make more sense to add an extra keyword argument weightless that can be true or false instead of absorbing this in the kind of statistic to compute? And support all iris statistics, like the other statistics functions do?

[schlunma]

This is now possible with the weights: false keyword in the recipe (see https://docs.esmvaltool.org/projects/ESMValCore/en/latest/recipe/preprocessor.html#examples).