Add PerAF derivative

[tsalo]

Branch for @joellebagautdinova to use.

Changes proposed in this pull request

• Add init_peraf_wf to calculate percent amplitude of fluctuation (PerAF; Jia et al., 2020).

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.64%. Comparing base (b42a2e5) to head (322e6d9).

Files with missing lines	Patch %	Lines
xcp_d/interfaces/restingstate.py	28.94%	27 Missing ⚠️
xcp_d/workflows/bold/cifti.py	66.66%	0 Missing and 3 partials ⚠️
xcp_d/workflows/bold/nifti.py	66.66%	0 Missing and 3 partials ⚠️
xcp_d/workflows/bold/connectivity.py	66.66%	0 Missing and 2 partials ⚠️
xcp_d/workflows/bold/outputs.py	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1587      +/-   ##
==========================================
- Coverage   82.87%   82.64%   -0.23%     
==========================================
  Files          58       58              
  Lines        7922     8027     +105     
  Branches     1130     1149      +19     
==========================================
+ Hits         6565     6634      +69     
- Misses       1015     1042      +27     
- Partials      342      351       +9     

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