Add .gitignore to model directories

[dweindl]

Add a .gitignore file to the amici-generated model directory to exclude the auto-generated files from version control.

Closes #2300.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (67d478c) 77.87% compared to head (8f7983a) 77.85%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2301      +/-   ##
===========================================
- Coverage    77.87%   77.85%   -0.03%     
===========================================
  Files          318      318              
  Lines        20529    20534       +5     
  Branches      1436     1436              
===========================================
- Hits         15987    15986       -1     
- Misses        4539     4545       +6     
  Partials         3        3              

Flag	Coverage Δ
cpp	73.63% <100.00%> (-0.03%)	⬇️
cpp_python	34.00% <100.00%> (+0.01%)	⬆️
petab	36.39% <100.00%> (+0.02%)	⬆️
python	72.41% <100.00%> (-0.03%)	⬇️
sbmlsuite	∅ <ø> (∅)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
python/sdist/amici/de_export.py	92.14% <100.00%> (+0.03%)	⬆️

... and 2 files with indirect coverage changes

[FFroehlich]

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