New aux data, new captions, selected for average validation score

[vdplasthijs]

What does this PR do?

• New aux data for testing.
• New captions with ~3 per concept, verified, see v5.json.
• Based on @gabrieletijunaityte script: R2 of linear probe of aux var encodings. Selected all aux- vars with R2 > 0.5 to be used for validation tracking, stored in v6.json (which is otherwise a copy of v5.json).

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