Function encoding issues

This issue is probably more for the STEP team looking at the standard. I am not sure whether they are looking into this repo but I don't know where else to post it. Pinging @TLiebich @arch1501

`RelPlacement\IfcLocalPlacement.RelativePlacement.Dim = 3` is bothering me. `IfcLocalPlacement.RelativePlacement` is an `IfcAxis2Placement` select on which `.Dim` is called. Obviously, the select does not have a `.Dim` attribute. This seems odd and in principle should not work. Any comments on this? I would offer myself to change to function.

Start by reviewing the reported expression `RelPlacement\IfcLocalPlacement.RelativePlacement.Dim = 3` and the surrounding IFC4.x specification context. Confirm with the STEP team whether the select access and proposed function change are valid, then document the agreed encoding and its expected behavior.

Written by the indexing model from the issue text.