Refactor DEModel, remove SbmlImporter dependency

[dweindl]

SbmlImporter should construct a DEModel, instead of DEModel importing an SbmlImporter.

Will allow moving DEModel to a separate module. Closes #2308.

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 77.76%. Comparing base (5e386ee) to head (e151b4b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2312      +/-   ##
===========================================
- Coverage    77.76%   77.76%   -0.01%     
===========================================
  Files          321      321              
  Lines        20538    20601      +63     
  Branches      1436     1436              
===========================================
+ Hits         15972    16020      +48     
- Misses        4563     4578      +15     
  Partials         3        3              

Flag	Coverage Δ
cpp	73.53% <76.31%> (-0.03%)	⬇️
cpp_python	34.12% <71.05%> (+0.09%)	⬆️
petab	36.56% <76.31%> (+0.15%)	⬆️
python	72.34% <82.89%> (+0.01%)	⬆️
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	93.71% <100.00%> (+0.01%)	⬆️
python/sdist/amici/de_model.py	93.33% <100.00%> (+0.04%)	⬆️
python/sdist/amici/splines.py	79.49% <ø> (ø)
python/sdist/amici/sbml_import.py	78.58% <80.64%> (+0.10%)	⬆️

... and 3 files with indirect coverage changes