stub out inherited methods of Akima1DInterpolator

As it stands, `Akima1DInterpolator.from_spline` does not create an Akima interpolant --- it just creates a pp representation of its spline argument instead. 
Both `from_spline` and `from_bernstein_basis` are artifacts of inheritance from PPoly, hence stub them out with NotImplementedErrors.
(cross-ref  gh-3742) 

Also cross-reference  Akima1D and Pchip interpolators in the docs, while I'm at it.