[misc] Add minor improvements

[felixdittrich92]

• Speed up polygon_iou
• use multiproc in core DetectionPostProc
• resolve hocr lang and geom in helpers
• Add corresponding tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.63%. Comparing base (7df5b1b) to head (41f17de).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2105      +/-   ##
==========================================
- Coverage   96.65%   96.63%   -0.03%     
==========================================
  Files         166      166              
  Lines        8702     8707       +5     
==========================================
+ Hits         8411     8414       +3     
- Misses        291      293       +2     

Flag	Coverage Δ
unittests	96.63% <100.00%> (-0.03%)	⬇️

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