Redefine TransportEvent in Consignment ABIE

Discussed at the Feb 12 TC meeting that the Consignment ABIE contains both a number of specific TransportEvent ASBIEs (ie., RequestedPickupTransportEvent, RequestedDeliveryTransportEvent, PlannedPickupTransportEvent, etc.) as well as a generic TransportEvent ASBIE. The definition of the generic TransportEvent should be updated to make it clear, that the ASBIE is only to be used for transport events where a specific ASBIE is not present.