[Feat] Provide derivatives for pow

[fjosw]

I explicitly added the derivatives for __pow__ and __rpow__, previously these were estimated via autograd. On my machine this results in a ~4x speedup for pow operations, obs ** 2 is now slightly faster than obs * obs.

I removed the Obs case from __rpow__ as this should be covered by __pow__ already and none of our tests fail 🤞

[fjosw]

We are still missing explicit derivatives for

• abs (I did not want to deal with the special cases)
• arcsin, arccos, arctan, arcsinh, arccosh, arctanh (probably hardly used at all)

[s-kuberski]

Great, thanks! I also think that it is not necessarily worth implementing the derivatives for the remaining functions. For __pow__ however, there might be some speedup in quite a number of real world examples.