Refactor the abstract class CreateConnectedElementI

[svengoldberg]

Description

In the current implementation of the CPACS Creator, the Wing and Fuselage class are inherited from an abstract class (cf. #1042). We want to avoid this behaviour and fix the code style by removing this class and adding a new std::variant type instead. In the actual code, it is then decided whether the current object is of type Wing or Fuselage and the respective member function is called.
Later on, the variant type can be extended by any other class (such as Duct, Pylon, Tank, ...), if the CPACS Creator should be able to add CPACS elements/sections within the according object.

The general procedure (and whether this is the best way) of course still can be discussed. Nonetheless, @joergbrech and I had a disscussion about it and agreed that using std::variant seems to be a good way to solve this code style issue.

Fix #1042

EDIT:

After some discussion, we agreed on discarding the std::variant way. Instead, a new (somehow) interface class is introduced which gets along without inheritance and therefore does not add some artifical structure layer.
In the class, the constructor has a template parameter to 'decide' the type of the incomming object

Task	Finished	Reviewer Approved
At least one test for the new functionality was added.	yes does not apply	OK
New classes have been added to the Python interface.	yes does not apply	OK
The code is properly documented with doxygen docstrings	yes does not apply	OK
Changes are documented at the top of ChangeLog.md	yes does not apply	OK

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.40%. Comparing base (2466e1d) to head (6b8166b).
Report is 11 commits behind head on cpacs_creator_dev_merge.

Additional details and impacted files

@@                   Coverage Diff                    @@
##           cpacs_creator_dev_merge    #1047   +/-   ##
========================================================
  Coverage                    71.40%   71.40%           
========================================================
  Files                          312      312           
  Lines                        28978    28976    -2     
========================================================
  Hits                         20691    20691           
+ Misses                        8287     8285    -2     

Flag	Coverage Δ
unittests	71.40% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/fuselage/CCPACSFuselage.h	100.00% <ø> (ø)
src/wing/CCPACSWing.h	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

[joergbrech]

LGTM! Thanks! I was gonna wait until CI works again with the merge, but

1. This is untested GUI code anyway, so no need to look at the coverage report
2. Waiting for CI to be fixed and then having to merge main back into this would be a little bit annoying.