Potential confusion in the direction of FFTs
In the current implementation of FFTOperator (https://gitlab.mpcdf.mpg.de/ift/nifty/-/blob/NIFTy_6/nifty6/operators/harmonic_operators.py#L76) we have the code:
if x.domain[self._space].harmonic: # harmonic -> position func = fft.fftn fct = 1. else: func = fft.ifftn fct = ncells
x is the input field.
This looks exactly the wrong way around.
Switching the FFT direction breaks almost none of our tests (only one, which already has a FIXME :).