Fix crash when trying to create flat multiscale graphs with >= 3 levels

[joeloskarsson]

Describe your changes

There is a a bug in

weather-model-graphs/src/weather_model_graphs/create/mesh/kinds/flat.py

Line 8 in 2456064

def create_flat_multiscale_mesh_graph(

that causes a crash whenever you try to create a multiscale graph by collapsing >= 3 levels of flat graphs. This is well-described in #33.

This PR fixes this bug and introduces a test for mesh methods that uses multiple levels, to check that they can handle more than 2 levels.

Issue Link

Fixes #33

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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[leifdenby]

LGTM! Thanks for fixing this :)