Revert fix in d3b12d81: FFTmax mean computation
Revert fix in d3b12d81: FFTmax mean computation

The signal of the whole population makes more by taking the mean of the
signals, and then applying the FFT.
Always check the synchrony indexes to see if the FFTmax makes sense.

neuro-lyon/multiglom-model,neuro-lyon/multiglom-model