Introduce a more strict schema for the data model of the statements

[GVogeler]

Currently the statements are rather lose. https://github.com/GVogeler/prosopogrAPhI/blob/master/prosopogrAPhI.yaml#L1107-L1111 provides some indications about the data model by listing URIs used in role and statementType properties. With this lose description of the data model using the interface for data creation in a database maintained by others is almost impossible. Creating a visual interface for data capture is also made very hard as the structure of the statements can only be inferred from all existing statements. To mitigate this, I'd propose to add a schema property to the describe endpoint that provides a JSON-schema for the data model. Preferably this should be a JSON-LD schema to create valid RDF data. The schema is considered complete, i.e. no other classes and properties would be allowed than defined in the schema.