Hi,

Do FFT method in autocorr use the real number optimization (only first half is calculated)?

In python numpy this is used with `rfft`

See http://eigen.tuxfamily.org/index.php?title=EigenFFT

```
Real FFT half-spectrum
1. Other libraries: use only half the frequency spectrum (plus one extra sample for the Nyquist bin) for a real FFT, the other half is the conjugate-symmetric of the first half. This saves a copy and some memory. The downside is the caller needs to have special logic for the number of bins in complex vs real.
2. How Eigen::FFT differs: The full spectrum is returned from the forward transform. This facilitates generic template programming by obviating separate specializations for real vs complex. On the inverse transform, only half the spectrum is actually used if the expected output type is real.
3. Use the Eigen::FFT::HalfSpectrum flag to change the default behavior
```