Improve spectrogram generation and add whitened data plot

I took a stab at improving the quality of the spectrogram plots we make as they are pretty uninformative currently.

We now plot an over-resolved whitened version, which I think is more similar to what we're used to looking at.

I also added a new plot of the whitened data in each of the interferometers so we can do at-a-glance whitening diagnoses.

Here are the plots for the gracedb example in the repository. The signal is now visible in the zoomed-in spectrogram.