Add trajectory distribution "layers"

[JosephCottam]

We loose some visual sense of density with the current trajectory visualizations vs just-plot-a-bunch-of-lines. Is there a way to get that sense back?

Idea: Plot a sequence of several quantiles on top of each other (95%, 50%, 20%). Only outline the outermost one. Let alpha-compose take care of making it look like a distribution. Take the sequence of intervals in as a parameter.