Clarify child Dataset relationships

[CAOShurong]

What changed

• replace the contradictory prohibition on child Dataset references with the relationship described in issue Property to link parent with child experiments #116
• distinguish a parent/child relationship from the root ./ import list
• add a minimal JSON-LD example showing a parent and child that are both selected for import

Why

The current specification first says that child datasets can be referenced and must also appear in the root import list, then says that datasets must not contain other datasets in hasPart. This makes the intended parent/child representation impossible to follow. The new wording keeps both roles explicit: nested hasPart records the relationship, while direct root membership marks a dataset for import.

Closes #116.

Validation

• git diff --check
• python -m pytest — 6 passed

[NicolasCARPi]

👌