Derivative computations for BicubicInterpolator's

Hi there !
I would like to improve the BicubicInterpolator (or the PiecewiseBicubicInterpolator) with the possibility to evaluate x, y, xy derivatives.

In the deprecated BicubicSplineInterpolator it was already possibile.

Before I start writing any lines of code I would like to know the rationale about removing this functionality.
Also, do you have any suggestion or guideline ?

I forked on github, I would like to contibute with a PR.

Thank you in advance