Property to link parent with child experiments

[simontaurus]

with #113 (seel also #98) changing the spec to

If a Dataset references other Datasets (e.g. a parent experiment with child experiments), they must also be present in the hasPart of ./ if they are meant to be imported. A Dataset MUST not contain other Datasets in its hasPart section.

ELNFileFormat is limited to option 2 / explicitly prohibits option 1 of the RO-Crate spec for nested directory structures

Directory hierarchies MAY be represented with nested Dataset Data Entities, or the Root Dataset MAY refer to files anywhere in the hierarchy using hasPart.

If hasPart is not allowed to link parent with child experiments, ELNFileFormat should define an alternative vocabulary term to do so.
One option would be isPartOf, pointing from the child to the parent element, which would at least to change the overall semantics in reference to option 1 ( hasPart pointing from the parent to the child element).