Consider specifying how a span placement works with regards to start and end

It was agreed that a "span placement" should use two IfcReferent/IfcRelPositions to provide a semantic definition of where a product starts and ends in a linear sense. However, there is no ordering of the referents for a product using this relationship.
Three alternatives without changing the schema:
- Always infer start and end from the referent nesting order
- Specify a use of IfcRelPositions.Name to indicate start and end (see e.g. https://standards.buildingsmart.org/IFC/RELEASE/IFC4_1/FINAL/HTML/link/ifcrelspaceboundary.htm)
- Defer to implementer or user agreements