Track lost degrees of freedom

[tsalo]

Closes #1296.

Changes proposed in this pull request

• Calculate lost degrees of freedom in LINC QC file.
• Fix up some of the documentation.

[codecov]

Codecov Report

Attention: Patch coverage is 43.33333% with 17 lines in your changes missing coverage. Please review.

Project coverage is 78.66%. Comparing base (14440d5) to head (452ab54).
Report is 17 commits behind head on dof.

Files with missing lines	Patch %	Lines
xcp_d/utils/modified_data.py	42.10%	11 Missing ⚠️
xcp_d/interfaces/utils.py	45.45%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dof    #1307      +/-   ##
==========================================
- Coverage   78.81%   78.66%   -0.15%     
==========================================
  Files          58       58              
  Lines        7223     7243      +20     
  Branches      951      952       +1     
==========================================
+ Hits         5693     5698       +5     
- Misses       1251     1266      +15     
  Partials      279      279              

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[tsalo]

Track remaining degrees of freedom (ignoring dfs from statistical tests)