ENH: Add start time attribute to PET model

[jhlegarreta]

Add start time attribute to PET model: allows to fit a PET model using the frames starting at frame_index.

Fixes #22.

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.28%. Comparing base (09df9e8) to head (c831ecd).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nifreeze/model/pet.py	91.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   77.01%   77.28%   +0.26%     
==========================================
  Files          28       28              
  Lines        1775     1796      +21     
  Branches      177      183       +6     
==========================================
+ Hits         1367     1388      +21     
  Misses        348      348              
  Partials       60       60              

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