Add Monotonic Cubic Spline Interpolation

[FoodMuncher]

Hi,

Thank you for this library! This is an amazing collection of functions and has been really helpful with my projects.

Would it be possible to add Monotonic Cubic Spline to the library?

This is the relevant scipy function: https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.PchipInterpolator.html#scipy.interpolate.PchipInterpolator

[josevalim]

We would accept pull requests that add it!