Phase shift of results compared to FFTW

[hv10]

This issue is to show that I am aware - but will not fix for now.

When applying bot FFTW and this package to the same array the representation of the FFT is different between them.
This is because of how I defined the twiddle factors. Flipping the sign before inv would fix it.

Applying ifft(fft(x)) with PureFFT gives the original array back.

AFAIK: As long as the inv of the twiddles is different for the forward and backwards pass it should work. I might by chance just have selected the opposite of FFTW.