Add Southern Ocean diagnostic and recipe

[thomaswilder]

Description

The changes made here are intended to form the start of a Southern Ocean diagnostic and recipe kit. The code will be built using Python and based on xarray. Previous diagnostics with similar functionality are written in ncl or python-iris. The diagnostics will be tailored towards models that use NEMO as the ocean component. This could be expanded in the future.

• Closes Add Southern Ocean recipe and diagnostic: transect with sigma2 #4111

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Checklist

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• 🛠 This pull request has a descriptive title
• 🛠 Code is written according to the code quality guidelines
• 🛠 Documentation is available
• 🛠 Tests run successfully
• 🛠 The list of authors is up to date
• 🛠 Any changed dependencies have been added or removed correctly
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New or updated recipe/diagnostic

• 🧪 Recipe runs successfully
• 🧪 Recipe is well documented
• 🧪 Figure(s) and data look as expected from literature
• 🛠 Provenance information has been added

New or updated data reformatting script

• 🛠 Documentation is available
• 🛠 The dataset has been added to the CMOR check recipe
• 🛠 The dataset has been added to the shared data pools of DKRZ and Jasmin by the @ESMValGroup/OBS-maintainers team
• 🧪 Numbers and units of the data look physically meaningful

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[valeriupredoi]

awesome, glad to see this shaping up nicely @thomaswilder 🍺 Pls let me know when you reckon it's ready for a start of the review process