[x] do we enfore a "namespace" for extra attributes? By have like "+Noise-attribute". This would avoid clashes between diff Extensions, but makes the whole thing more complex.
[x] where are the extensions files? If they are in a subfolder of schemas /extensions/ then it means that for CityJSON types to be reused one must write have relative path ../cityobjects.json
[x] rename schemas to cityjson.schema.json, like glTF does?
[x] make a schema to validate a cityjson extension
/extensions/then it means that for CityJSON types to be reused one must write have relative path../cityobjects.jsoncityjson.schema.json, like glTF does?