SBML import: support == operator

[dweindl]

Adds support the the equality operator in piecewise conditions or event triggers.

Closes #2685.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.86%. Comparing base (9a90c24) to head (2e10b75).

Files with missing lines	Patch %	Lines
python/sdist/amici/import_utils.py	50.00%	2 Missing ⚠️
python/sdist/amici/sbml_import.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2708      +/-   ##
===========================================
- Coverage    77.93%   77.86%   -0.07%     
===========================================
  Files          326      326              
  Lines        23009    23014       +5     
  Branches      1479     1479              
===========================================
- Hits         17931    17920      -11     
- Misses        5067     5083      +16     
  Partials        11       11              

Flag	Coverage Δ
cpp	74.72% <40.00%> (-0.07%)	⬇️
cpp_python	33.59% <20.00%> (-0.01%)	⬇️
petab	39.39% <20.00%> (-0.01%)	⬇️
python	72.92% <40.00%> (-0.07%)	⬇️

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

Files with missing lines	Coverage Δ
python/sdist/amici/sbml_import.py	78.65% <0.00%> (-0.08%)	⬇️
python/sdist/amici/import_utils.py	80.80% <50.00%> (-0.64%)	⬇️

... and 4 files with indirect coverage changes

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