refactor: Simplified implementation of StepperExtensionList

[benjaminhuth]

In this PR I propose a simplified implementation of the StepperExtensionList. I'm currently working with it, and I found it first quite difficult to look through the structure, and then also not so handy to change things (must be always changed both in implementation and the interface class...)

With this proposal, I reimplemented the same functionality with some fold expressions and lambdas, and at least to me it is now a lot clearer what is done here, and everything is in one file.

[codecov]

Codecov Report

Merging #817 (55bfe42) into main (31faea5) will decrease coverage by 0.00%.
The diff coverage is 53.12%.

❗ Current head 55bfe42 differs from pull request most recent head 624df01. Consider uploading reports for the commit 624df01 to get more accurate results

@@            Coverage Diff             @@
##             main     #817      +/-   ##
==========================================
- Coverage   48.64%   48.63%   -0.01%     
==========================================
  Files         328      327       -1     
  Lines       16900    16906       +6     
  Branches     7935     7944       +9     
==========================================
+ Hits         8221     8223       +2     
+ Misses       3080     3079       -1     
- Partials     5599     5604       +5     

Impacted Files	Coverage Δ
...e/include/Acts/Propagator/StepperExtensionList.hpp	59.52% <53.12%> (-21.43%)	⬇️
Core/include/Acts/Propagator/Navigator.hpp	57.54% <0.00%> (ø)
Core/include/Acts/Utilities/BinningData.hpp	66.27% <0.00%> (ø)
Core/include/Acts/Propagator/AtlasStepper.hpp	69.36% <0.00%> (ø)
Core/include/Acts/Propagator/EigenStepper.hpp	67.56% <0.00%> (ø)
Core/include/Acts/Utilities/BoundingBox.ipp	43.93% <0.00%> (+5.12%)	⬆️

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[paulgessinger]

Yup, good idea. I think it's effectively exactly the same, but you can write it a lot more compact this way and there's no need to add extra types.

I think most of us (or me at least) are not super comfortable with fold expressions yet, but this is the way.