ENH: add reconstruction filtering support with --rec-label

[mnoergaard]

This PR addresses issue #291 to allow for filtering of different PET reconstructions as indicated by the 'rec' entity prior to running the PET workflows.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.50%. Comparing base (aff6ed3) to head (5fd6438).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
+ Coverage   83.21%   83.50%   +0.28%     
==========================================
  Files          87       87              
  Lines        8304     8382      +78     
  Branches      742      887     +145     
==========================================
+ Hits         6910     6999      +89     
+ Misses       1167     1160       -7     
+ Partials      227      223       -4     

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