[IFC Rail] Spatial containment of IfcSpatialZone

How do we _place_ `IfcSpatialZone` within the Spatial structure.

Two solutions are in my opinion feasible:

* Using `IfcRelContainedInSpatialStructure` with the idea that `IfcSpatialZone` are like `IfcProduct` that need to be contained within the spatial structure. Problem is that it somehow change the meaning of the relation (no schema change needed, it si teh solution as documented in [SpatialDecomposition](https://github.com/bSI-InfraRoom/IFC-Specification/blob/develop/IFC4x3/Templates/Object%20Composition/Aggregation/Spatial%20Decomposition/Documentation.md))
* Using `IfcRelReferencedInSpatialStructure` with the idea that `IfcSpatialZone` are not properly contained in a spatial structure. Problem is that the same relation will be used to link the `IfcSpatialZone` to the upper (the spatilastructure) and the lower (the Element attached to the SpatialZone), still no schema change needed...

Example provided from \[IFC Rail\] Urban Railway Story Line cabling below:



Ping @TLiebich, @SergejMuhic and @larswik !