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

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.23%. Comparing base (b51c487) to head (d433a31).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #560      +/-   ##
==========================================
+ Coverage   75.09%   75.23%   +0.14%     
==========================================
  Files          38       38              
  Lines        4578     4600      +22     
==========================================
+ Hits         3438     3461      +23     
+ Misses       1140     1139       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tsalo]

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