Derivative in k-space, transition dipoles

[stevenbos123]

We implement the derivative in k-space with a finite difference method. I think we can have a look at this sisl methods https://sisl.readthedocs.io/en/latest/_modules/sisl/physics/state.html#StateC.derivative

[stevenbos123]

@rreho What is this usefull for?

[rreho]

In general, any formula that might contain derivatives with respect to k. Right now, it applies mainly to topological observables.