chaos vis colorization improvements

[katestange]

Chaos visualizer could let the user configure a gradual fade on the dots, so that the picture has a dynamic quality (not just filling in more and more).

[gwhitney]

Just adding on to avoid issue proliferation:

• [ ] There is a bug in the parameter interface in that if you choose "highlight by walker" and then choose an invalid walker in the resulting dropdown, but then choose a different coloring scheme, the error message that you have chosen an invalid walker persists and the visualizer won't draw, even though now that setting is both invisible and irrelevant. When the parameter box is scrolled down so you don't see the error message, it's really hard to figure out this is going on!
• [ ] There is currently no way to make a monochrome image if you have multiple walkers; none of color by walker, index, destination, or highlight walker does it. Suggest adding a monochrome option with a color picker.