Add PySBModel for handling of PySB models

[dweindl]

Draft for handling PySB models in PEtab.

• adds newly introduced model IDs in yaml file to petab.Model, and handles change form sbml_files to model_files
• adds support for the newly introduced model<->petab entity mapping file

[codecov-commenter]

Codecov Report

Merging #145 (91e5de4) into develop (751b628) will decrease coverage by 1.82%. The diff coverage is 57.50%.

@@             Coverage Diff             @@
##           develop     #145      +/-   ##
===========================================
- Coverage    78.07%   76.25%   -1.82%     
===========================================
  Files           32       34       +2     
  Lines         2915     3155     +240     
  Branches       690      764      +74     
===========================================
+ Hits          2276     2406     +130     
- Misses         461      552      +91     
- Partials       178      197      +19     

Impacted Files	Coverage Δ
petab/lint.py	72.20% <14.28%> (-2.35%)	:arrow_down:
petab/problem.py	64.51% <27.27%> (-2.79%)	:arrow_down:
petab/parameters.py	84.53% <57.14%> (-2.68%)	:arrow_down:
petab/mapping.py	57.89% <57.89%> (ø)
petab/observables.py	97.26% <60.00%> (-2.74%)	:arrow_down:
petab/models/pysb_model.py	60.80% <60.80%> (ø)
petab/models/model.py	68.33% <64.28%> (-3.67%)	:arrow_down:
petab/models/sbml_model.py	88.40% <66.66%> (-7.96%)	:arrow_down:
petab/yaml.py	85.88% <66.66%> (-1.93%)	:arrow_down:
petab/C.py	100.00% <100.00%> (ø)
... and 5 more

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