Add inverse FFT

Mikhail Svechnikovrequested to merge
m.fftw into main

Now FourierTransform class can do:

  1. rfft: from real signal NxM to complex spectrum Nx(M/2+1)
  2. ramplitude: from real signal NxM to real amplitudes NxM
  3. irfft: from complex spectrum Nx(M/2+1) to real signal NxM
  4. fftshift and ifftshift: shift low frequency to the center of Fourier array or back to the corner

All this will is needed to generate roughness map.

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