Correct frequency to have standard ordering

[tjjarvinen]

Autocorrelation function is calculated using

tmp = [ fft(abs2.(ifft(r))) for r in eachrow(μ)]

which seems to have ifft and fft the opposite way.

The resulting spectrum is correct, because it is corrected when frequency variable is taken out in spectrum-function.
But it migh be a good idea to correct this in future.

[tjjarvinen]

For a reference look equation (1) in https://doi.org/10.1039/C3CP44302G