ENH: Implement squared exponential covariance function

[jhlegarreta]

Implement squared exponential covariance function.

[jhlegarreta]

Transfer from nipreps/eddymotion#261.

[codecov]

Codecov Report

❌ Patch coverage is 39.47368% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.63%. Comparing base (dc42626) to head (ade3c35).

Files with missing lines	Patch %	Lines
src/nifreeze/model/gpr.py	39.47%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
- Coverage   84.46%   82.63%   -1.83%     
==========================================
  Files          37       37              
  Lines        2156     2194      +38     
  Branches      243      246       +3     
==========================================
- Hits         1821     1813       -8     
- Misses        296      343      +47     
+ Partials       39       38       -1     

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