Fix AttributeError during model import

[dweindl]

Previously, importing a model module would result in an AttributeErrorif no other objects were imported from amici.sim.sundials beforehand.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.36%. Comparing base (9d4ab62) to head (a1c853a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3200      +/-   ##
==========================================
- Coverage   77.05%   76.36%   -0.69%     
==========================================
  Files         317      317              
  Lines       20903    20903              
  Branches     1484     1484              
==========================================
- Hits        16106    15962     -144     
- Misses       4789     4933     +144     
  Partials        8        8              

Flag	Coverage Δ
cpp	71.54% <ø> (-0.06%)	⬇️
cpp_python	36.71% <ø> (ø)
petab	46.78% <ø> (ø)
python	68.30% <ø> (-0.05%)	⬇️
sbmlsuite-jax	?

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see 8 files with indirect coverage changes

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