symmetrize the Sassena intermediate structure factor to negative times

[jmborr]

Sassena output contains the intermediate structure factor for non-negative times. Prior to taking the Fourier transform, we have to symmetrize it to negative times.[[BR]]

We have to slightly modify the LoadSassena algorithm to do this
The classical intermediate scattering function obeys F_(q,t)=F(q,t). Splitting into real and imaginay parts:[[BR]]
F=Re+iIm leads to[[BR]]
Re_(q,t)=Re(q,-t) and Im*(q,t)=-Im(q,-t)

[jmborr]

This issue was originally trac ticket 5534