Refactor symbolic event processing

[dweindl]

Extract some neutral changes from #1539 to reduce the diff there and avoid conflicts.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.33%. Comparing base (6261491) to head (a309b8b).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
python/sdist/amici/de_model.py	85.71%	1 Missing ⚠️
python/sdist/amici/import_utils.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2538      +/-   ##
===========================================
- Coverage    77.76%   76.33%   -1.44%     
===========================================
  Files          325      320       -5     
  Lines        21047    20892     -155     
  Branches      1473     1473              
===========================================
- Hits         16367    15947     -420     
- Misses        4653     4921     +268     
+ Partials        27       24       -3     

Flag	Coverage Δ
cpp	73.50% <85.71%> (+0.03%)	⬆️
cpp_python	34.04% <64.28%> (+0.01%)	⬆️
petab	?
python	72.33% <85.71%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
python/sdist/amici/de_model.py	92.53% <85.71%> (-0.10%)	⬇️
python/sdist/amici/import_utils.py	88.37% <85.71%> (ø)

... and 23 files with indirect coverage changes