Clean up amici.sim.sundials exports

[dweindl]

• Re-export import_model_module.
• Set __all__

The goal should be that after having compiled a model, for most applications, the only required import for loading and simulating that model is amici.sim.sundials.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.41%. Comparing base (4ca828f) to head (ee52cfc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3082      +/-   ##
==========================================
- Coverage   78.07%   77.41%   -0.66%     
==========================================
  Files         312      312              
  Lines       20494    20496       +2     
  Branches     1500     1500              
==========================================
- Hits        16000    15868     -132     
- Misses       4485     4619     +134     
  Partials        9        9              

Flag	Coverage Δ
cpp	71.46% <100.00%> (+<0.01%)	⬆️
cpp_python	37.51% <100.00%> (+<0.01%)	⬆️
petab	46.29% <100.00%> (+<0.01%)	⬆️
petab_sciml	13.87% <100.00%> (+0.01%)	⬆️
python	70.10% <100.00%> (+<0.01%)	⬆️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
python/sdist/amici/sim/sundials/__init__.py	85.71% <100.00%> (+2.38%)	⬆️

... and 5 files with indirect coverage changes

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