add study descriptor as seperate term

[stap-m]

Description of the issue

We have data descriptor and model descriptor. We also use study descriptors for the scenario bundles. The latter are annotated in OEO with the annotation property 'oekg annotation' value 'study descriptor'. But we didn't actually define study descriptor in oeo explicity.

Ideas of solution

Make study descriptor subclass of data descriptor:
Proposal for a definition: A study descriptor is a data descriptor that allows the annotation of study information within a scenario bundles by means of keywords from ontological terminology.

Workflow checklist

• I discussed the issue with someone else than me before working on a solution
• I already read the latest version of the workflow for this repository
• The goal of this ontology is clear to me

I am aware that

• every entry in the ontology should have a definition
• classes should arise from concepts rather than from words