Fix wrong number of mesh levels when grid is multiple of refinement factor

[joeloskarsson]

Describe your changes

There is currently a bug where you can not make hierarchical graphs if the grid dimensions are an exact multiple of the refinement factor. This is best illustrated with an example:

• If you have a 27 x 27 grid
• grid_refinement_factor=3 and level_refinement_factor=3

Then you should be able to have a first mesh level of 9x9 and a second of 3x3 mesh nodes. This does currently not work, as the code return that the maximum number of possible mesh levels is 1, rather than 2.

We tracked down this issue to the casting to int in

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

Lines 116 to 121 in 1d1407b

	max_mesh_levels = (
	(np.log(coord_extent) - np.log(grid_refinement_factor))
	/ np.log(level_refinement_factor)
	).astype(
	int
	) # (2,)

Due to the numerical precision of this calculation it sometimes ends up with something like 1.999999999999996 rather than exactly 2. This then gets floored to 1.

This PR fixes this by adding a tiny epsilon before doing this rounding. It also adds a test for this.

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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[ErikLarssonDev]

Nice work!

[sadamov]

This looks good to me. Easy fix for numerical issues. Alternatively this also solves the isssue: coord_extent = np.array((xy.shape[2], xy.shape[1]), dtype="float32")
But, I am fine with the epsilon approach and from my side this PR can be merged.

[joeloskarsson]

Thanks, that is good to know! That might mean that we no longer need the epsilon in #32. Since coord_extent is being changed in #32 I am thinking that we just merge this as a fix for now and then I'll take another look if the epsilon is still needed after that change is done.