fix: correctly dump Breit-Wigner

[redeboer]

There was a bug when dumpin an AmplitudeModel containing a RelativisticBreitWigner to YAML or XML. This PR fixes it.

Note: better solution is through #383, but this is more of a hot-fix that is required for TensorWaves, see e.g.
https://github.com/ComPWA/tensorwaves/pull/179/checks?check_run_id=1454030793

[codecov]

Codecov Report

Merging #395 (b0d33a8) into master (8e9d6f0) will increase coverage by 0.02%.
The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   90.44%   90.47%   +0.02%     
==========================================
  Files          26       26              
  Lines        3599     3610      +11     
  Branches      874      877       +3     
==========================================
+ Hits         3255     3266      +11     
- Misses        172      173       +1     
+ Partials      172      171       -1     

Flag	Coverage Δ
unittests	90.47% <56.25%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
src/expertsystem/io/_xml/_build.py	85.35% <ø> (ø)
src/expertsystem/io/_xml/_dump.py	87.85% <50.00%> (+0.23%)	⬆️
src/expertsystem/io/_yaml/_dump.py	85.34% <57.14%> (+1.23%)	⬆️