IfcClassification has a flattened HasReferences hierarchy upon export

The `HasReferences` attribute denotes the classification hierarchy, up through a tree of `IfcClassificationReference` before the top-level `IfcClassification` entity.

Revit does not seem to export the subset of the classification tree upon export, but instead flattens the classification hierarchy so that only the leaf nodes are included.

The practical downside is that if an element is classified as "Foo-Bar-Baz", a search for elements under "Foo-Bar" will turn up nothing in the IFC file.

No files or tests are named in the issue. Start by tracing the export handling for IfcClassification and IfcClassificationReference, then inspect an exported IFC containing a nested Foo-Bar-Baz classification. Done means the exported HasReferences hierarchy preserves parent classifications so searches for Foo-Bar find the classified element.

Written by the indexing model from the issue text.