area_weights for cordex in area_statistics

[mwjury]

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Closes {Link to corresponding issue}

This is pull request is depending on #184

[valeriupredoi]

cool! cheers @mwjury - just a couple minor comments 🍺

[valeriupredoi]

oh and can you pls add a test case 🍺

[mwjury]

@valeriupredoi I added the tests, could you have look please.

[valeriupredoi]

looks good, cheers muchly @mwjury 🍺

[valeriupredoi]

ready by me, @mattiarighi would be okay to test - I think it's good to go w/o testing anyway 🍺