Add method for spline from "left-edge"

[florence-bockting]

The idea here would be to add a class method to SplineScipy, something like from_normal_coefficients (terrible name, I can't think of how to name it better). The idea would be that, you given in say, "[1, 0]" and a domain of [-10, 20] and it translates the coefficients so they will work with scipy (so the coefficients would need to be "[1, -10]" in this trivial case). It's super annoying to always have to do this translation to x=0 being the left-edge of the domain by hand every time, this class method would try and solve that.

Originally posted by @znichollscr in #20 (comment)