Bug: a88/b1b

Ideally all the plotting classes should be useable headless, without
having to instantiate a PuffinApp or create any windows. A couple of
plots still make reference to PuffinApp; there are some other 
dependencies on classes in the puffinplot.window package. Plots
make use of GraphDisplay, which subclasses JPanel. It's not 
immediately obvious how much work it would take to decouple plots
fully from Swing, but it's definitely worth looking into. Especially
for publication, being able to script a plot is a strong selling
point. It might be possible now in a desktop environment with a 
redundant PuffinPlot main window, but obviously headless scripting
would be greatly preferable.