rationalise the use of IfcPropertySet for all properties / unify the current separation between property sets and quantity sets

There are two separate structures for exchanging property sets and quantity sets. But in reality it is often difficult to determine, what is a property and what is a quantity. And both structures are 90% identical.

Merging both into an enhanced definition of `IfcPropertySet `would remove potential duplication and redundency and simplify the schema without a loss of functionality.

Start by comparing the current property-set and quantity-set structures referenced in the issue, then trace how IfcPropertySet is defined and used. Define the enhanced IfcPropertySet approach and verify that merging the structures preserves the functionality currently provided by both.

Written by the indexing model from the issue text.