Refactor: PEtab code to sub-package

[dweindl]

Move PEtab-related code to a new amici.petab subpackage. An attempt to organize the growing codebase a bit better. More to be done. Unless anybody is using private functions, this should not break anything.

More parts to be moved separately. No change in functionality.

[codecov]

Codecov Report

Merging #2205 (2879cf8) into develop (ffcbf66) will decrease coverage by 0.05%.
The diff coverage is 76.09%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2205      +/-   ##
===========================================
- Coverage    53.80%   53.75%   -0.05%     
===========================================
  Files           34       38       +4     
  Lines         5652     5659       +7     
===========================================
+ Hits          3041     3042       +1     
- Misses        2611     2617       +6     

Flag	Coverage Δ
petab	53.75% <76.09%> (-0.05%)	⬇️
sbmlsuite	?

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

Files	Coverage Δ
python/sdist/amici/__init__.py	95.55% <ø> (ø)
python/sdist/amici/petab/__init__.py	100.00% <100.00%> (ø)
python/sdist/amici/petab_import.py	76.12% <ø> (+7.87%)	⬆️
python/sdist/amici/petab_import_pysb.py	63.52% <ø> (ø)
python/sdist/amici/petab_objective.py	91.66% <ø> (+2.01%)	⬆️
python/sdist/amici/petab_util.py	0.00% <0.00%> (-89.66%)	⬇️
python/sdist/amici/petab/util.py	89.28% <89.28%> (ø)
python/sdist/amici/petab/conditions.py	96.58% <96.58%> (ø)
python/sdist/amici/petab/cli/import_petab.py	0.00% <0.00%> (ø)
python/sdist/amici/petab/parameter_mapping.py	73.79% <73.79%> (ø)

[FFroehlich]

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