Implement general kinetic model with weighted average-based priors

[tsalo]

Builds on #545.

Changes proposed in this pull request

• For multi-PLD PCASL data, calculate CBF and ATT using the weighted average approach, then pass those arrays into the GKM method as initial estimates for those variables.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.33%. Comparing base (32cc579) to head (811d291).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
aslprep/utils/cbf.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #560      +/-   ##
==========================================
+ Coverage   76.19%   76.33%   +0.14%     
==========================================
  Files          38       38              
  Lines        4382     4404      +22     
==========================================
+ Hits         3339     3362      +23     
+ Misses       1043     1042       -1     

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[tsalo]

Using the weighted-average CBF and ATT as priors doesn't seem to help in our test dataset.