Centralize default model path generation

[dweindl]

Currently, there are several places where default model directory paths are generated. Centralize that.

Running the test suite or building the documentation creates model directories all over the place. Reduce that.

Allow overriding the model base dir via environment variable.

Having version-specific subdirectories circumvents import issues with models from different amici versions.

Closes #2994.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.04%. Comparing base (cb1f08e) to head (8c3fe3e).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
python/sdist/amici/__init__.py	91.66%	1 Missing ⚠️
python/sdist/amici/de_export.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3002      +/-   ##
==========================================
- Coverage   76.65%   76.04%   -0.61%     
==========================================
  Files         309      309              
  Lines       19891    19898       +7     
  Branches     1498     1498              
==========================================
- Hits        15248    15132     -116     
- Misses       4630     4753     +123     
  Partials       13       13              

Flag	Coverage Δ
cpp	72.53% <73.33%> (+0.05%)	⬆️
cpp_python	37.71% <26.66%> (+<0.01%)	⬆️
petab	38.61% <88.88%> (+0.03%)	⬆️
petab_sciml	14.14% <13.33%> (+<0.01%)	⬆️
python	69.36% <86.66%> (+0.06%)	⬆️
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/petab/petab_import.py	100.00% <ø> (ø)
python/sdist/amici/petab/sbml_import.py	75.21% <ø> (+0.61%)	⬆️
python/sdist/amici/pysb_import.py	94.49% <100.00%> (+0.01%)	⬆️
python/sdist/amici/sbml_import.py	81.47% <100.00%> (-12.49%)	⬇️
python/tests/conftest.py	43.33% <100.00%> (+4.04%)	⬆️
python/sdist/amici/__init__.py	95.23% <91.66%> (-0.85%)	⬇️
python/sdist/amici/de_export.py	96.89% <0.00%> (ø)

... and 6 files with indirect coverage changes

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