Add a widget to set the y limits on the eigenvalue plot

[moorepants]

The caster line (lower blue one in this plot) often makes the lower y limit very large and the more interesting parts of the graph are not shown. It would be nice to both have a better default limits (maybe -10 to 10 would suffice for most bikes) and helpful to allow the user to change the limits. We change the x limits via the speed slider. We could add a slider for the y limits or maybe simple text boxes are enough. Here's an example of a plot that isn't zoomed like we want:

But fixing #78 would allow the user to dynamically zoom, so that is a better long term solution for changing the axes.