ENH: Implement squared exponential covariance function

[jhlegarreta]

Implement squared exponential covariance function.

[jhlegarreta]

Transfer from nipreps/eddymotion#261.

[codecov]

Codecov Report

Attention: Patch coverage is 39.47368% with 23 lines in your changes missing coverage. Please review.

Project coverage is 63.77%. Comparing base (6b6ba70) to head (aae6307).

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   65.84%   63.77%   -2.07%     
==========================================
  Files          18       18              
  Lines         931      969      +38     
  Branches      119      122       +3     
==========================================
+ Hits          613      618       +5     
- Misses        274      308      +34     
+ Partials       44       43       -1     

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