FIX: Ensure BOLD file count is preserved

[mgxd]

Fixes #82

Grouping should not increase/decrease the number of BOLD files.
Conditions that modify combinations/files should be exclusive.

[codecov-commenter]

Codecov Report

Merging #83 (d6eed49) into master (14304f6) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   31.55%   31.54%   -0.01%     
==========================================
  Files          42       42              
  Lines        3255     3256       +1     
==========================================
  Hits         1027     1027              
- Misses       2228     2229       +1     

Impacted Files	Coverage Δ
nibabies/utils/bids.py	23.15% <0.00%> (-0.25%)	⬇️

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