change ifft to fft in gate frequency domain calculation

[jhillairet]

To address #390 , changed
kernel=fft.fftshift(fft.ifft(fft.ifftshift(gate, axes=0), axis=0))
to
kernel2=fft.ifftshift(fft.fft(fft.fftshift(gate, axes=0), axis=0))

While it produces a different mathematical result (for example due to the different iFFT/FFT normalization, see fft doc), the final result is the same as only the normalized magnitude is kept after:

kernel =abs(kernel).flatten() # take mag and flatten
kernel=kernel/sum(kernel) # normalize kernel