As a breaking change, this replaces `specimen_of`, which served this
exact purpose, but only for `Subject`-`Subject` relationships.
It is now also added as a property to `DataItem`. This enables
describing the linkage between a `DataItem` and a `Subject` when
no information on the generating process is available (`generated_by`).
This avoid the requirement to introduce "empty" `StudyActivity` record
in order to express `DataItem`-`Subject` relationships in this case.
