Improve whitening plot

Fix broken utilities producing a whitened waveform reconstruction. This is now down using band-passing, windowing and the FD PSD. It is meant only as an illustrative tool (does not use exactly the same ACF entering the likelihood), but it is nevertheless useful to visualise the result. Example of new plot attached for GW150914 with a rough and simplified estimate of noise (which is why the signal is underestimated).