ENH: Add flag for STC reference time

[mgxd]

This ports over nipreps/fmriprep#2540, allowing for control of the slice timing reference used during correction.

Additionally, add SliceTimingCorrected and StartTime fields to the BOLD metadata

[codecov-commenter]

Codecov Report

Merging #137 (2c8bb44) into master (23c3bef) will decrease coverage by 0.00%.
The diff coverage is 32.25%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
- Coverage   32.58%   32.58%   -0.01%     
==========================================
  Files          44       44              
  Lines        3609     3637      +28     
==========================================
+ Hits         1176     1185       +9     
- Misses       2433     2452      +19     

Impacted Files	Coverage Δ
nibabies/cli/parser.py	2.32% <0.00%> (-0.17%)	⬇️
nibabies/workflows/bold/stc.py	30.43% <12.50%> (-4.86%)	⬇️
nibabies/config.py	57.32% <100.00%> (+0.13%)	⬆️
nibabies/workflows/bold/outputs.py	25.86% <100.00%> (+4.76%)	⬆️

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