Ratio plots

[GilesStrong]

In HEP it is useful to compare 'collider data' to 'MC data' as a ratio plot. These are normally placed under more informative plots, e.g.:

It would be useful to have something similar for LUMIN, like a subplot that sits under the main plot, but I'm not sure how easy it would be to implement in a generalising way for all plots. For now it will probably be sufficient to extend plot_binary_sample_feat to include a ratio plot of background to collider data, or signal to background (and to extend it to plot collider data as dots, and uncertainty bands for background and collider data). It's not a priority, though, since most analyses will probably still produce final plots in ROOT.