handle relation inside group in refercrosswalk

[floriangantner]

Is your feature request related to a problem? Please describe.
The current ReferCrosswalk templating items to cerif xml cannot handle relations in metadatagroup, only simple textual metadata values. The metadata inside this groups are often authority controlled and the entitiy behind the value often contains additional informations useful for the crosswalk.handle some related item inside some metadatagroup

Describe the solution you'd like
We would like to handle some related (authority controlled) item inside some metadatagroup in the cerif crosswalk. This allows adopters to extend the ReferCroswalk (e.g. cerif crosswalk,) with additional metadata from the related item, e.g. some identifier of persons or orgunits.

Describe alternatives or workarounds you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
We will provide some draft of our solution / extension of the refercrosswalk.