[FEATURE] Write solid unit tests for CompositionModel

[jwa7]

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[Luthaf]

Could you expand a bit where the current tests are lacking?

[PicoCentauri]

I don't know exactly, but I discussed with @jwa7 offline and I suggested to do some very simple test cases where you read of the compositions weights. For example a system with only on atom type. For this system the weights are just the average energy. One could do this also for two atom types and so on.