Constructor for an interpolation which is the derivative of another Interpolation

Currently we have `hessian` and `gradient` to evaluate derivatives of `Interpolation` objects. However, sometimes we wish to simply construct a whole *new* interpolation which is the derivative of another and use this.
(also, more generally, we might want to construct interpolations which are integrals of other interpolations, but there are a couple of other details there).

It looks like this should be mathematically not too complex, but I got lost trying to understand how this would get the correct order and boundary conditions.