Citygml 3.0 spaces are not supported

[mlavik1]

Models containing "spaces" (from CityGML 3.0) will fail to import. Examples being gen:GenericOccupiedSpace.

See OGC docs: https://docs.ogc.org/guides/20-066.html#ug-coremodel-section

Here's a sample model (converted form CityJSON):
cube.city.zip

I'll start working on support for this and send you a PR, if you don't mind :)
From what I can see it should be pretty simple to do. Just a matter of adding some new node types, and extend the if-checks in cityobjectelementparser.cpp.
Update: Ok, so probably a bit more than that, when including the new additions to "transport", that depend on this..