DM: Add direct connection from Shipment to ULD

[nscheiber-champ]

Freight forwarders may deliver their shipment already on ULDs.

The DM currently features no mechanism to unambiguously indicate that: Using Piece + Composing + UnitComposition + ULD indicates that a Piece may be on an ULD when it is delivered, but not that this was contracted as such.

Hence, I propose to add the properties

• Shipment:loadingUnits
• LoadingUnit:ofShipment

A shipment of ULDs would then hold links to all Pieces and all ULDs as delivered by the freight forwarder to the airline.

[lambertciata]

Can it be linked to #265 ?

[lambertciata]

We can reject this one as we move forward with #265