Implement missing features of the json-schema datamodel parser

A first experimental version of the json-schema parser has been implemented with #35 (closed). However, the parser still lacks a variety of features that are necessary or at least very helpful in creating a datamodel from more or less arbitrary json schemas:

• inheritance: This will probably require an extension of datamodel itself, so we won't implement it
• suggested properties: ditto
• $defs and $ref in json schema
• Re-use of already defined RTs or Poroperties as List properties
• Reference properties that are different from the referenced RT. (Although this is possible for list of references)
• Values
• Roles
• The extern keyword from the yaml parser
• Documentation
• We currently require the root element of each json-schema to be a RecordType (or Property) with type and title. In generic schemas, only its properties might actually be the datamodel.