Add documentation and verification routine(s) for plotter benchmark

[lam2mo]

Here are some correctness conditions from Pavel, from most to least granular:

• Each function evaluation must be accurate (versus a true complex-number evaluation).
• The computed colors for each pixel are exactly correct (colors are represented by non-approximate integers).
• The colors be close but not necessarily exact.
• The colors are close in a perceptual sense (say, gamma-corrected and transformed to Lab color).
• The colors are smoothly changing.