Support for custom base quantities export in IFC2x3

According to this thread on the old Sourceforge forum the exporter supports (used to support?) adding userdefined (i.e. driven by formulas) base quantities to loadable families: https://sourceforge.net/p/ifcexporter/discussion/general/thread/c5ab88d7/

I tested this with a generic model family for the ```NetVolume``` quantity (using the ```IfcQtyNetVolume``` parameter from the shared parameter file that ships with the exporter) in IFC2x3 CV2.0 but without success. In IFC4, on the other hand, base quantities are exported for ```IfcBuildingElementProxy``` elements *without* having to resort to custom QTO parameters at all. Which is nice, of course, but since in this particular case we owe the client IFC2x3 it won't help much.

Am I missing something here or is this a regression?

- Tested in 23.2.4.0
- File to reproduce: https://github.com/andydandy74/MyIssues/raw/master/RevitIFC594/118_IFC_CustomQTOParameters.rvt

Start with the supplied Revit file, the IFC2x3 CV2.0 export path, and the shared IfcQtyNetVolume parameter mentioned in the report; compare its result with the IFC4 export behavior. Done means the user-defined NetVolume base quantity is exported correctly for the IFC2x3 case, or the regression is explained.

Written by the indexing model from the issue text.