Add new line ratios, new graphs vs radius

- Add the strong line ratios so we can make the BPT diagram
- Comment on some possible problems with the binning (on reflection,
  none of them look serious)
- Calculate T, n from the line ratios of the binned images (we don't use
  these yet)
- Produce a lot of graphs of histograms of things versus radius:
  1. The ratios 5755/6583 and 6716/6731
  2. The surface brightnesses in various lines (note I want to keep Ha
     but swap the other line surf brightnesses for the BPT ratios)

Reviewed-by: William Henney <will@henney.org>