API: add additional output format RDFS
Please add RDFS as additional output format. Reasons are:
- It is a common format e.g. CIDOC CRM uses it too
- It is a good opportunity for refactoring to keep data and output format separate which will be helpful for future format additions
- think about how the URL should look like, e.g. we talked about something like "/api/0.1/entity/105.rdf"
- add an rdfs link in util.py at add_system_data (in a new branch)
Feel free to ask anytime if questions arise, maybe we try to communicate with notes in this issue.