Relabel support for Darknet (YOLO) annotation files

[monocongo]

Resolves issue #137

[codecov]

Codecov Report

Merging #138 into master will increase coverage by 0.26%.
The diff coverage is 63.41%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   55.72%   55.99%   +0.26%     
==========================================
  Files          17       17              
  Lines        1127     1168      +41     
==========================================
+ Hits          628      654      +26     
- Misses        499      514      +15

Impacted Files	Coverage Δ
tests/test_relabel.py	59.75% <57.69%> (-0.96%)	⬇️
src/cvdata/relabel.py	59.74% <73.33%> (+3.28%)	⬆️

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