Fix incorrect initialization for non-constant initial assignments

[dweindl]

Fixes a bug where targets of initial assignments are not initialized correctly if the initial assignment expression is implicitly time-dependent. Fixes #2936.

• eliminate state variables from x0 after replacing rate-of expressions
• implement supposed-to-be static expressions that have non-constant entities in their initial condition as state variables instead of expressions.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.02%. Comparing base (678f7a3) to head (0868fa6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
python/sdist/amici/sbml_import.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2939      +/-   ##
==========================================
- Coverage   79.57%   79.02%   -0.56%     
==========================================
  Files         362      362              
  Lines       23157    23174      +17     
  Branches     1588     1588              
==========================================
- Hits        18428    18313     -115     
- Misses       4717     4849     +132     
  Partials       12       12              

Flag	Coverage Δ
cpp	74.96% <95.65%> (+0.01%)	⬆️
cpp_python	33.15% <69.56%> (+0.02%)	⬆️
petab	38.81% <60.86%> (+0.02%)	⬆️
python	73.81% <95.65%> (+0.01%)	⬆️
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/de_model.py	92.50% <100.00%> (-0.37%)	⬇️
python/sdist/amici/sbml_import.py	80.16% <92.30%> (-11.88%)	⬇️

... and 4 files with indirect coverage changes

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