Revit can export many IfcSite objects that aren't creating the spatial hierarchy

If an object in a file, say a bollard or street furniture, is assigned to the Revit site category, then an IfcSite entity is created for it on export.

In essence, this means that the `IfcSite` is treated as an `IfcElement` as part of the [Spatial containment](https://standards.buildingsmart.org/IFC/DEV/IFC4_2/FINAL/HTML/link/spatial-containment.htm) concept. This is incorrect, as an `IfcSite` is an `IfcSpatialElement`.

So if I search an IFC file looking for IfcSite and expecting to see it part of the spatial tree, instead, I find single objects...

Start by reproducing the Revit export described in the issue with an object assigned to the Revit site category, then inspect the resulting IFC spatial containment against the linked IFC concept. Done means those objects no longer produce standalone IfcSite entities treated as elements, and the exported IfcSite hierarchy is valid.

Written by the indexing model from the issue text.