Add scipy.interpolate API

[f0uriest]

Adds a number of classes that replicate most of the functionality of the corresponding classes from scipy.interpolate:

• scipy.interpolate.PPoly -> interpax.PPoly
• scipy.interpolate.Akima1DInterpolator -> interpax.Akima1DInterpolator
• scipy.interpolate.CubicHermiteSpline -> interpax.CubicHermiteSpline
• scipy.interpolate.CubicSpline -> interpax.CubicSpline
• scipy.interpolate.PchipInterpolator -> interpax.PchipInterpolator

Resolves #6
Resolves #19

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.36842% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 97.92%. Comparing base (3c66c30) to head (0fa0231).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   91.56%   97.92%   +6.35%     
==========================================
  Files           6        7       +1     
  Lines         759      965     +206     
==========================================
+ Hits          695      945     +250     
+ Misses         64       20      -44     

Files	Coverage Δ
interpax/__init__.py	100.00% <100.00%> (ø)
interpax/_fd_derivs.py	99.52% <100.00%> (+23.44%)	⬆️
interpax/_spline.py	97.73% <100.00%> (ø)
interpax/utils.py	87.50% <100.00%> (+20.83%)	⬆️
interpax/_ppoly.py	96.93% <96.93%> (ø)