feat: Added more flexible matrix subscripting

[marberts]

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[codecov-commenter]

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Codecov Report

❌ Patch coverage is 87.01299% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.45%. Comparing base (68e00f1) to head (6908911).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
R/extract.piar_index.R	82.75%	10 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   94.03%   93.45%   -0.59%     
==========================================
  Files          32       32              
  Lines        1208     1252      +44     
==========================================
+ Hits         1136     1170      +34     
- Misses         72       82      +10     

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