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 0% with 38 lines in your changes missing coverage. Please review.

Project coverage is 9.79%. Comparing base (628aa8f) to head (fd91c82).

Files with missing lines	Patch %	Lines
src/nifreeze/model/gpr.py	0.00%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #36       +/-   ##
==========================================
- Coverage   69.15%   9.79%   -59.37%     
==========================================
  Files          20      20               
  Lines         963    1001       +38     
  Branches      119     122        +3     
==========================================
- Hits          666      98      -568     
- Misses        254     903      +649     
+ Partials       43       0       -43     

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