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
where x
is the input field.
This looks exactly the wrong way around.
@parras, @pfrank, @kjako, @reimar, @veberle, what do you think?
Switching the FFT direction breaks almost none of our tests (only one, which already has a FIXME :).