Super-issue containing the full restructuring of the current Masterdata definitions (object types and vocabularies) for an ontology-informed schema.

• Define their properties.
• Define more objects for Entity up to level 4
• Define their properties
• Define application-specific objects for Activities (level 3 and more) and Entities (level 4-5 and more)

Besides, we need:

• General changes to make it easier to work with the schema in Python
• Refactor the to_rdf API
• Define rules for the schema (e.g., max depth level of inheritance)