Generate Code for Ducts and adapt existing code base to it

[joergbrech]

Description

Fixes #888.

This is a preparation PR for #881. This PR includes the new ducts definition in the CPACS schema and the generated code. It adapts all relevent TiGL functions to it so that the code compiles and all unit and integration tests succeed.

The refactoring of the non-generated code boils down to making sure, that certain structural elements, as well as fuselage positionings, sections and segments do not automatically assume to be children of fuselages, since now they can belong to either a duct or a fuselage.

The actuall implementation of the ducts is not part of this PR. The CCPACSDuct::BuildLoft function exists but will error.

How Has This Been Tested?

All existing unit and integration tests are successfull.

Screenshots, that help to understand the changes(if applicable):

Checklist:

• A test for the new functionality was added.
• All tests run without failure.
• The new code complies with the TiGL style guide.
• New classes have been added to the Python interface.
• API changes were documented properly in tigl.h.

[codecov-commenter]

Codecov Report

Merging #887 (8551f55) into master (8b0a2ae) will decrease coverage by 0.18%.
The diff coverage is 51.00%.

@@            Coverage Diff             @@
##           master     #887      +/-   ##
==========================================
- Coverage   62.53%   62.34%   -0.19%     
==========================================
  Files         292      296       +4     
  Lines       24640    24750     +110     
==========================================
+ Hits        15408    15430      +22     
- Misses       9232     9320      +88     

Impacted Files	Coverage Δ
src/CCPACSPositionings.cpp	90.32% <0.00%> (-4.60%)	⬇️
src/ducts/CCPACSDuct.cpp	0.00% <0.00%> (ø)
src/ducts/CCPACSDuctStructure.cpp	0.00% <0.00%> (ø)
src/fuselage/CCPACSDoorAssemblyPosition.cpp	0.00% <0.00%> (ø)
src/fuselage/CCPACSFuselage.h	100.00% <ø> (ø)
src/fuselage/CCPACSFuselageSections.cpp	50.00% <0.00%> (-7.15%)	⬇️
src/fuselage/CCPACSFuselageSegment.h	50.00% <ø> (ø)
src/fuselage/CCPACSFuselageStringer.cpp	0.00% <0.00%> (ø)
...c/fuselage/CCPACSFuselageStringerFramePosition.cpp	4.06% <0.00%> (+0.06%)	⬆️
src/fuselage/CCPACSSkin.cpp	0.00% <0.00%> (ø)
... and 23 more