Avoid events for smooth piecewise

[FFroehlich]

Summary

• detect continuously differentiable piecewise expressions
• check smoothness w.r.t. given parameters only
• keep these piecewise expressions instead of converting to Heaviside
• extend tests for constant piecewise expressions

Testing

• pre-commit run --files python/sdist/amici/import_utils.py python/tests/test_heavisides.py
• pytest python/tests/test_heavisides.py::test_parse_piecewise_c1_no_heaviside python/tests/test_heavisides.py::test_parse_piecewise_discontinuous_to_heaviside python/tests/test_heavisides.py::test_parse_piecewise_c1_constant_zero -q

fixes #2049

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https://chatgpt.com/codex/tasks/task_b_685d399bee14832b899b26d9136e2fda

[codecov]

Codecov Report

Attention: Patch coverage is 78.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 79.05%. Comparing base (167864b) to head (8e9ca87).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
python/sdist/amici/import_utils.py	75.55%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2840      +/-   ##
==========================================
- Coverage   79.62%   79.05%   -0.57%     
==========================================
  Files         358      358              
  Lines       23033    23077      +44     
  Branches     1581     1582       +1     
==========================================
- Hits        18339    18243      -96     
- Misses       4683     4823     +140     
  Partials       11       11              

Flag	Coverage Δ
cpp	74.98% <68.00%> (+<0.01%)	⬆️
cpp_python	32.50% <68.00%> (-0.68%)	⬇️
petab	38.90% <12.00%> (-0.09%)	⬇️
python	73.84% <78.00%> (+<0.01%)	⬆️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
python/sdist/amici/pysb_import.py	94.57% <ø> (ø)
python/sdist/amici/sbml_import.py	82.12% <100.00%> (-12.07%)	⬇️
python/sdist/amici/sbml_utils.py	81.52% <100.00%> (+0.11%)	⬆️
python/sdist/amici/import_utils.py	80.97% <75.55%> (-3.98%)	⬇️

... and 8 files with indirect coverage changes

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[FFroehlich]

Comment on why this hasn't been merged yet: I currently don't really trust the C1 continuity check as assignments etc are not flattened out. This might imply that we could inadvertently miss discontinuities, but maybe it's all fine as we can assume continuity of the respective terms. Haven't thought about it enough.