Requirements for superelevation transition pivot point

This issue provides requirements for superelevation transition pivot point. See #269 discussion on Pset_Superelevation.

As shown in the attached image the pivot point, or line of rotation, for a superelevation transition can be at the centerline or one edge of the pavement. More generally, the rotation can occur at any point in a cross section.

Secondary lines for the pivot point could be specified, however this is unnecessarily complex. Superelevations are defined with Pset_Superelevation for an IfcAnnotation and the IfcAnnotation is is placed using an IfcLinearPlacement for a specific IfcAlignment. The Pset would need to reference one IfcAlignment while it's owning IfcAnnotation whould need to reference a different IfcAlignment.

@larswik - requirement per your request in #269. We can discuss in detail as needed.