Performs the FFT thickness analysis and returns the obtained thickness.
Parameters:
n_spec: number of the spectrum in the spectrum list
wn_min: wavenumber minimum of the used spectral range
wn_max: wavenumber maximum of the used spectral range
fft_np: number of data points used to sample the spectral range
d_min: expected thickness minimum
d_max: expected thickness maximum
n: refractive index of the layer
VisualBasic example: scout.fft_thickness(1,650,7000,2048,5,50,2.0)