Fix handling of initial assignments containing splines

[dweindl]

Currently, x0 must not depend on splines. Therefore, substitute the spline symbol by its piecewise expression when compiling x0 expressions.

Reported by @m-philipps.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.01%. Comparing base (17dbad1) to head (5f8824f).
⚠️ Report is 113 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2943      +/-   ##
==========================================
- Coverage   79.59%   79.01%   -0.59%     
==========================================
  Files         362      362              
  Lines       23175    23180       +5     
  Branches     1588     1588              
==========================================
- Hits        18446    18315     -131     
- Misses       4717     4853     +136     
  Partials       12       12              

Flag	Coverage Δ
cpp	75.73% <100.00%> (+0.77%)	⬆️
cpp_python	33.15% <20.00%> (-0.01%)	⬇️
petab	38.80% <20.00%> (-0.01%)	⬇️
python	73.80% <100.00%> (-0.02%)	⬇️
sbmlsuite-jax	?

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	80.26% <100.00%> (-11.87%)	⬇️

... and 7 files with indirect coverage changes

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[m-philipps]

Thanks a lot!