provide a macro "PUMLAInjectChildRelations(xyz)

[DrMarkusVoss]

Is your feature request related to a problem? Please describe.
Similar as the "InjectChildElements". When modeling the standard example for the C4
model, the need for this arose, as I also want to model re-usable relations that are
related to the embedding context, like here:
PUMLAC4System_Boundary(techtribes, "techtribes.js") { PUMLAInjectChildElements(techtribes) PUMLAInjectChildRelations(techtribes) }

Describe the solution you'd like
That means, not only the macro must be added, but also relations need to have a "parent" attribute to be dealt with.

Describe alternatives you've considered
Not providing it would be a solution, but with what reasoning can't I have child relations, when I can have child elements?