- Add ability to fill-up the fields of the Holding in the event context.
- Use Holding schema in the Instance schema
- Implement the Writer interface
- Write fields marked as "Map allowed" https://docs.google.com/spreadsheets/d/17kCccovBR-RAVK-coW7fpXvJfaRmYA2mUHoVLSF9Spo/edit#gid=451506657
- Cover with tests
- HoldingWriter class that fills up fields of the Holding in Inventory instance