drop reid_metadata tuple return from deidentify()

[tclose]

Implementations now only strip identifying data and return the deidentified fileset; callers that need to know what changed (e.g. for a re-identification audit trail) diff metadata before/after instead. Also parallelises DicomCollection.deidentify() across a thread pool via a new max_workers kwarg.

[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.20%. Comparing base (0517862) to head (e5a5cb6).

Files with missing lines	Patch %	Lines
extras/fileformats/extras/medimage/dicom.py	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   77.68%   78.20%   +0.51%     
==========================================
  Files          29       29              
  Lines         753      748       -5     
==========================================
  Hits          585      585              
+ Misses        168      163       -5     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.