Add smoothing spline

[f0uriest]

Resolves #3

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 7.69231% with 24 lines in your changes missing coverage. Please review.

Project coverage is 95.56%. Comparing base (850d846) to head (37cb50a).
Report is 105 commits behind head on main.

Files with missing lines	Patch %	Lines
interpax/_ppoly.py	7.69%	24 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   97.92%   95.56%   -2.37%     
==========================================
  Files           7        7              
  Lines         965      991      +26     
==========================================
+ Hits          945      947       +2     
- Misses         20       44      +24     

Files with missing lines	Coverage Δ
interpax/__init__.py	100.00% <ø> (ø)
interpax/_fd_derivs.py	99.52% <ø> (ø)
interpax/_ppoly.py	86.48% <7.69%> (-10.46%)	⬇️

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