refactor!: remove XML support

[redeboer]

Keeping up support for the old ComPWA format is becoming too bothersome with regard to #384 and #385. Eventually, we have to abandon XML support anyway, because we want to dump to and from dict structures and leave the file format choice (e.g. YAML or JSON) up to the user.

[codecov]

Codecov Report

Merging #425 (c8aea3c) into master (11e4154) will increase coverage by 0.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
+ Coverage   89.53%   90.30%   +0.77%     
==========================================
  Files          26       22       -4     
  Lines        3745     3426     -319     
  Branches      927      842      -85     
==========================================
- Hits         3353     3094     -259     
+ Misses        200      169      -31     
+ Partials      192      163      -29     

Flag	Coverage Δ
unittests	90.30% <100.00%> (+0.77%)	⬆️

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

Impacted Files	Coverage Δ
src/expertsystem/io/__init__.py	95.00% <100.00%> (-0.66%)	⬇️
src/expertsystem/io/_yaml/validation.py	100.00% <100.00%> (ø)
src/expertsystem/reaction/conservation_rules.py	92.15% <0.00%> (+0.58%)	⬆️
src/expertsystem/amplitude/helicity_decay.py	90.31% <0.00%> (+0.77%)	⬆️
src/expertsystem/io/_yaml/_dump.py	81.89% <0.00%> (+1.72%)	⬆️
src/expertsystem/io/_yaml/__init__.py	84.84% <0.00%> (+6.06%)	⬆️

[spflueger]

👍