BUG fixed in FFTGF: fft from iw to tau (fftgf_iw2tau) had an error in the
evaluatin of the tail at last point, generating a problem when Ntau > Liw.
Fixed and checked.
NB: the evaluation of the density (G(tau=0-)) does not improve with Ntau,
but with Liw, provided by user. Best usage is Ntau=Liw.