Make DEModel._sparsesyms symbols sympy.Symbols

[dweindl]

Previously, sparse matrix symbols in DEModel._sparsesyms were strings, as opposed to other model "syms" which were sympy.Symbol. Make that consistent.

Fixes #1061.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.98%. Comparing base (34043c6) to head (218c1a6).
⚠️ Report is 51 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2894      +/-   ##
==========================================
- Coverage   79.55%   78.98%   -0.57%     
==========================================
  Files         362      362              
  Lines       23106    23104       -2     
  Branches     1587     1587              
==========================================
- Hits        18381    18248     -133     
- Misses       4714     4845     +131     
  Partials       11       11              

Flag	Coverage Δ
cpp	74.90% <100.00%> (-0.01%)	⬇️
cpp_python	33.18% <100.00%> (-0.01%)	⬇️
petab	38.89% <100.00%> (-0.01%)	⬇️
python	73.76% <100.00%> (-0.01%)	⬇️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
python/sdist/amici/cxxcodeprinter.py	95.57% <100.00%> (ø)
python/sdist/amici/de_export.py	97.23% <100.00%> (-0.02%)	⬇️
python/sdist/amici/de_model.py	92.43% <100.00%> (-0.43%)	⬇️
python/sdist/amici/de_model_components.py	91.27% <ø> (-1.75%)	⬇️

... and 4 files with indirect coverage changes

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