add new container field to Etikett model
add new container field to Etikett model

- the container field is used to express lists and sets
in json-ld in order to help converters to transform json to rdf
- the current etikett master supports the new etikett format too

edoweb/regal-api,jschnasse/regal-api,jschnasse/regal-api,hbz/lobid-rdf-to-json,edoweb/regal-api,jschnasse/regal-api,jschnasse/regal-api,edoweb/regal-api,edoweb/regal-api