Clarify Fraction in IfcMaterialConstituent

In [IfcMaterialConstituent](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialConstituent.htm) the **Fraction** attribute is indicated to be optional and can represent both _volume_ or _weight_ as a Normalised Ratio Measure (a 0-1 number).

In a complex object, fraction of weight and fraction of volume will lead to very different outcomes.

There is no further clarification at the IfcMaterialConstituentSet either.

[IfcMaterialConstituentSet](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialConstituentSet.htm)

Start by reading the linked IfcMaterialConstituent and IfcMaterialConstituentSet definitions in the IFC4x3 specification. Determine whether Fraction represents volume, weight, or requires additional clarification, then update both definitions so the intended interpretation is unambiguous.

Written by the indexing model from the issue text.