Make FourierOp differentiable wrt trajectory

[fzimmermann89]

Our FourierOp should be differentiable with respect to the trajectory. This is quite easy to do, see for example https://arxiv.org/abs/2111.02912

Currently we rely on torchkbnufft or adjoint-is-backward to do the backward for a Nufft.

We could solve this by creating a way to force a nufft, and either improving torchkbnufft or switching to finufft, that already has the appropriate backward implementations.
Or, by providing a custom backward over the whole FourierOperator that also takes the trajectory into account. This would then work for any kind of fourieroperation and we would not have to think about what the fourierop does internally.