Rename QC metrics and output QC files as TSVs

[tsalo]

Closes none.

Changes proposed in this pull request

• Rename the QC metrics from camelCase to snake_case, per BIDS convention.
• Output the QC files as TSVs instead of CSVs, per BIDS convention.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (f66ed4c) 79.02% compared to head (1dbd64f) 78.98%.

Files	Patch %	Lines
aslprep/cli/aggregate_qc.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #375      +/-   ##
==========================================
- Coverage   79.02%   78.98%   -0.05%     
==========================================
  Files          39       39              
  Lines        4157     4149       -8     
  Branches      602      602              
==========================================
- Hits         3285     3277       -8     
  Misses        676      676              
  Partials      196      196              

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