This should make quite a few integration tests unnecessary.  for example it allows
`test_reference_options()` in `test_json_schema_exporter.py`.

Also moved it into a class.

F schema export references

See merge request !81

F simple schema export

See merge request !80

Merge branch 'f-schema-export-references' of gitlab.indiscale.com:caosdb/src/caosdb-advanced-user-tools into f-schema-export-references