Allow adding properties to existing classes in other dictionaries

Currently, in order to invent my own property Foobar that is assigned to IfcWall, I have to additionally invent my own class (with related IFC entity set to IfcWall).

I understand there is logic why this is a good thing (i.e. makes dictionaries self contained for maintenance) but I also think this leads to fragmentation, not standardisation (which is the point of bSI).

I would love to be able to "extend" another dictionary's class with my own properties instead of redefining every IFC class again.

The issue names no files, tests, or entry points. Start by reviewing the repository's documentation and examples to understand how classes, dictionaries, and related IFC entities are represented; done means defining and documenting an agreed way to extend a class from another dictionary without redefining it.

Written by the indexing model from the issue text.