Visualize whole tree of infections

[afmagee42]

Resolves #24 and #15. Solves #53 along the way because the broken plots that led to were driving me crazy.

Code lives in ringvax.plot, and I'm rather tempted to wrap it all up into a plotting object since that would avoid a lot of unneeded passing of objects around (the simulation, plotting parameters, and the plot itself).

Colors and styles can and probably should be argued over. I strove to keep things clean and visible first and foremost, but also to use our color palette and keep things accessible. I think this is mostly achieved, except perhaps marking detections which is a bit ugly. I tried delineating detection types by color, instead of shape, but I ran out of colors that were both easy to see on both latent and exposed backgrounds and which were truly distinctive from each other.

To keep some of the plotting-induced post-processing sane, I added infection attributes: the id and the infectees.