Allowing shape reuse through IfcProductDefinitionShape and potential simplifications of mapping representations

Originally brought up [here](https://forums.buildingsmart.org/t/is-ifcproductdefinitionshape-a-1-1-relationship/2746)

IfcProductDefinitionShape relates back to IfcProduct with this data type: SET [1:?] OF [IfcProduct](http://localhost:5000/IFC/RELEASE/IFC4x3/HTML/lexical/IfcProduct.htm) FOR Representation

In other words, there are now more unofficial ways to reuse geometry:

1. Through reusing the IfcRepresentationMap (the official way via the mapped geometry concept)
2. Through reusing the IfcRepresentationItem (I assume this is not semantic, reuse simply means optimisation, not actual instancing)
3. Through reusing the IfcShapeRepresentation (also, I assume not semantic, reuse simply means optimisation, not instancing)
4. Through reusing the IfcProductDefinitionShape (is this now semantic, or is it just an optimisation thing?)

I personally propose that there should be one official way to reuse geometry - and that's through the official mapped geometry concept.

Can someone confirm that this SET[1:?] is _not_ semantic? i.e. it doesn't mean the product is instancing geometry? If not, then it should be official and a concept should probably be created?

Review the linked forum discussion and the IFC4x3 definitions for IfcProductDefinitionShape, IfcProduct, IfcRepresentationMap, IfcRepresentationItem, and IfcShapeRepresentation. The work is complete when the semantic status of shape reuse is decided and the specification change or clarification needed for the accepted approach is documented.

Written by the indexing model from the issue text.