ENH: Implement squared exponential covariance function

[jhlegarreta]

Implement squared exponential covariance function.

[codecov]

Codecov Report

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

Project coverage is 64.18%. Comparing base (f10fc12) to head (77968f3).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
- Coverage   66.24%   64.18%   -2.06%     
==========================================
  Files          19       19              
  Lines         942      980      +38     
  Branches      119      122       +3     
==========================================
+ Hits          624      629       +5     
- Misses        274      308      +34     
+ Partials       44       43       -1     

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[oesteban]

I believe we want to leverage sklearn's composition of kernels on this one (and kill to birds with one stone)