Update analyse_power_spectrum.m

If you need to shift frequency components to be visually clearer (not necessary always), use fftshift after fft.
FFT => F(X) = Sum(a*cos(nx)+j*b*sin(nx)), each term is exp(2 * pi *j *w * t) = cos(2*pi*w*t) + j sin(2*pi*w*t)
abs of the complex (a+bi) is to sqrt(a^2+b^2).