Incorrect gmx analyze -ac -nonormalize
The gmx analyze -ac output is correct, but with -nonormalize it's orders of magnitude too small. It's still correct in 5.0
Fix non-normalization of FFT autocorr
The function many_auto_correl incorrectly divided by the number of
data points, leading to incorrect absolute values of autocorrelation
functions, as e.g. out by gmx analyze -ac -nonormalize.
Also simplified the FFT size calculation.
Added a test for -nonormalize functionality.