A complex property's or quantity's members.
The entity type: IFCPROPERTYSINGLEVALUE, IFCQUANTITYLENGTH, ...
The unit the property states; resolve it with IfcModel.resolveUnit.
value: the typed value; enumerated/list: a list; reference: a ref.
The declared IFC value type, e.g. IFCLENGTHMEASURE.
One property, quantity or predefined-set attribute.