Make `TransportType` abstract

[Robbendebiene]

TransportType is declared abstract in Transmodel. As far as I understand it acts as a base type for VehicleType, Train etc.

Also something like ref="TransportType" does not exist anywhere in the NeTEx schema. So there is also no way to define a TransportType.

[skinkie]

I think the Dummy is the Abstract here.

[Robbendebiene]

True but that is because it is a _DummyType. At least it doesn't affect the TransportType which is why I can do this

<vehicleTypes>
  <TransportType>
	...  
  </TransportType>
</vehicleTypes>