Fix data splitting for S2BMS UC

[gabrieletijunaityte]

What does this PR do?

Clean up datasplits, add their explanations. Prepares datasplits (S2BMS and unlabelled) that:

• Share val-test portions.

• Training set is filtered based on data availability per modality.

• Additionally some missed before test fixes.

Before submitting

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
• Did you test your PR locally with pytest command?

[vdplasthijs]

Very minor; I think the explanations txt file should be updated? Please go ahead and merge when it's ready!