Reference from ocpTT to platformEdge

As already stated in the forum (1) an optional reference from the 'ocpTT' to the newly introduced platform edge element is needed.

• If the 'stopPostRef' is given, the 'platformEdgeRef' is redundant.
• If 'stopPostRef' and/or 'platformEdgeRef' are given, the 'trackRef' is redundant.

It would be nice to put this reference in the 'stopDescription' element together with other stop characteristics.

(1) http://www.railml.org/forum/ro/?group=2&id=286