[FIX] clarify that <physio|stim>.json is REQUIRED

[sappelhoff]

crossref:

• https://github.com/bids-standard/bids-validator/issues/1024
• Cleaning up BIDS examples that showcase use of physiological and other continuous recordings bids-examples#186

I think my proposed change here is already implicit in the specification.

When a TSV is supplied, a JSON MUST be included as well.

Because in _physio or _stim files, the file headers of the TSV are stripped and are shipped in the sidecar JSON.

So if a TSV is supplied but no JSON, the TSV becomes meaningless, because nobody knows about the columns anymore.

This will also have to be checked in the validator bids-standard/bids-validator#1024

[melanieganz]

Thanks @sappelhoff, I am fine with that clarification.