Make GenericTransmissionUnits "box" level elements

* Fixes export of schedules in DSE perspective
* Previously defined marker interface IPlatformArchitectureElement is
  still applied via base type "TransmissionUnit"
* As a reference: OffChipNetworks in the generic architecture are also
  declared as "box" level elements

Issue-Ref: 4089
Issue-Url: #4089

Signed-off-by: Simon Barner <barner@fortiss.org>
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