The isSerialCompoundLink relation relates a (end-to-end) Link to a series of Links (segments).

Proposal: allow that the list contains either LinkGroups or Links.

Rationale: Imagine that a given segment only describes a linkgroup. It is currently only possible to describe the serial
 compound link by first defining the individual Link. This proposal makes it easier to describe the serial compound link
 at the expense of precision (if a segment is a LinkGroup, it is not obvious which of the Links in the LinkGroup is used
).