Sbml model enhancements

[fbergmann]

This PR allows to create SBML models from string, but more critically considers an SBML model with warnings still as valid SbmlModel. The current PEtab implementation considers a file with warnings as invalid SBML, which is not the case.

[dweindl]

Thanks, looks good to me.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 74.86%. Comparing base (9a4efb4) to head (195f534).

Files with missing lines	Patch %	Lines
petab/v1/models/sbml_model.py	33.33%	4 Missing ⚠️
petab/v1/sbml.py	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #333      +/-   ##
===========================================
- Coverage    74.92%   74.86%   -0.07%     
===========================================
  Files           52       52              
  Lines         4913     4921       +8     
  Branches       845      847       +2     
===========================================
+ Hits          3681     3684       +3     
- Misses         923      928       +5     
  Partials       309      309              

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