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 :).