bib: Add Sadavoy et al. 2013.

The paper has been published since I used it for my ADASS poster.
Add introduction to the bad-baseline section.

Take out the red ink too.
Rework the bad-baseline introduction.

Replace the high-frequency example with one that shows the three main
types.  There are a couple more rarer forms.  I still need to replace
the low-frequency examples, not least because one is cropped at the
top.
Update and expand the high-frequency interference section.
Update the section on the non-linear baselines.
Minor correction to the bad-baseline results subsection.

Mainly to cite Sadavoy et al.
Add figures showing high-frequency interference profiles.
Change a figure caption.

The plot was not as I originally intended, once I'd had made something.
In the process it did unearth a couple of bugs in my primitive.