IndexSeries: no enforcement that at least one of indexed_timeseries or indexed_images is present #703

Summary

IndexSeries defines two links, both optional (quantity: '?'):

• indexed_timeseries — target: ImageSeries (discouraged, to be deprecated)
• indexed_images — target: Images (preferred)

The NWB schema language has no native "at least one of" constraint, so an IndexSeries with neither link is technically schema-valid but semantically meaningless — there is nothing to index into.

Proposed short-term fix

Strengthen the IndexSeries type-level doc string to explicitly state that at least one link must be present, and that indexed_images is the preferred target. This does not change schema validation but makes the requirement clear to users and tool authors.

Proposed long-term fix

Once indexed_timeseries is formally deprecated in the schema:

1. Remove indexed_timeseries (or mark it deprecated with deprecated: true if the spec supports it).
2. Make indexed_images required (quantity: 1), eliminating the ambiguity entirely.

Related

The indexed_images link doc also contains a stale field name ('ordered_images' instead of 'order_of_images'), fixed in PR #695.