STY: Disallow None for mandatory PET model parameters

[jhlegarreta]

Disallow None for mandatory PET model timepoints and xlim parameters.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.90%. Comparing base (09df9e8) to head (8f60db3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #318      +/-   ##
==========================================
- Coverage   77.01%   76.90%   -0.12%     
==========================================
  Files          28       28              
  Lines        1775     1775              
  Branches      177      177              
==========================================
- Hits         1367     1365       -2     
- Misses        348      349       +1     
- Partials       60       61       +1     

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