Some modifications in ducts definition

[joergbrech]

During the implementation of ducts in TiGL (https://github.com/DLR-SC/tigl/pull/889) we made some modifications in the current ducts definition. Firstly, we now have ducts and ductAssemblys at the same level under the ductsType and ductAssemblys reference a duct via UID. This way, ducts can be reused by more than one ductAssembly. Finally, we reordered the elements in two xsd:all elements to fix some issues with the order of reading the CPACS files via the routines provided by https://github.com/RISCSoftware/cpacs_tigl_gen.

Concretely,

• ducts element in configuration before fuselage and wing type fixes issue in tigl
• ducts are defined on the same level as ductAssemblies to make them reusable
• ductAssemblies reference ducts via UID to make ducts reusable
• sections element in ductType before segments fixes issue in tigl

addresses #768

[MarAlder]

Having some difficulties in resolving conflicts. I manually aligned with your implementation via 00d643d and will close this merge request.