ENH: add tracer-label functionality for filtering

[mnoergaard]

This PR addresses issue #189 by

1. Added a tracer label execution setting and CLI option to target PET processing to specific tracers in line with participant/session filters.

2. Propagated tracer labels into PET BIDS filters and validate requested tracers against available data before running workflows.

3. Introduced parser tests covering tracer-specific filtering behavior and error handling for unavailable tracers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.76%. Comparing base (133526d) to head (a61d19b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
+ Coverage   80.63%   80.76%   +0.13%     
==========================================
  Files          84       84              
  Lines        6512     6557      +45     
  Branches      657      662       +5     
==========================================
+ Hits         5251     5296      +45     
  Misses       1097     1097              
  Partials      164      164              

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