When getCurveSet() is called on an unsupported entity, it runs into a
NameError exception because elts is not defined. Instead print a message
and return gracefully.
This is in line with overall behaviour of the importer: Ignore
unsupported elements instead of thowing errors.
- Can be based on an edge like normal wall
- Now has a 'host' property to embed into another object (tree only)
- Mullions have different height and width properties
- Mullions or panels can be disabled
