test: add tests for image preparation conversion

[stijnmoreels]

Add tests for the image preparation conversion.

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[stijnmoreels]

Still missing a single code execution path for the property tests, and that's the path where the image preparation uses the np.dot function, but I'm having trouble understanding both what it does and why the current implementation uses a fixed value: [0.3, 0.59, 0.11].

[stijnmoreels]

And apparently, the tests now fail bc the matplotlib doesn't have an Axes class anymore with numColls properties. 🤔 Wondering if we use a fixed version during testing.