[WIP] Three equations model for sloshing simulations

[jmark]

This is a draft PR for the three equations model suitable, e.g., for tank sloshing simulations. As of now, this PR is primarily a backup and not ready for review. Nevertheless, interested readers are welcome to check it out and give feedback.

[github-actions]

Review checklist

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Code quality

• The code can be understood easily.
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Documentation

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Testing

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Performance

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Verification

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Created with ❤️ by the Trixi.jl community.

[DanielDoehring]

Xref: https://www.mi.uni-koeln.de/NumSim/2024/01/26/snapshot-2d-sloshing-simulation/