Add PySBModel.to_str

[dweindl]

Convenient for debugging.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.18%. Comparing base (97bf770) to head (ebfbafb).

Files with missing lines	Patch %	Lines
petab/v1/models/pysb_model.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
==========================================
- Coverage   75.18%   75.18%   -0.01%     
==========================================
  Files          62       62              
  Lines        6848     6850       +2     
  Branches     1223     1223              
==========================================
+ Hits         5149     5150       +1     
- Misses       1232     1233       +1     
  Partials      467      467              

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