More blending modes for uncertainty

[mcorrell]

Right now uncertainty is encoded by blending to white in LAB space (more uncertainty, more whiteness). We call this mode usl (for "uncertainty-saturation-lightness").

We might want more modes. For instance, desaturating to denote uncertainty. Or just lightening to denote uncertainty. Or interpolating to a third color (that may or may not be white) to denote uncertainty.

A more user-friendly way of selecting these modes other than just passing a string to a case statement somewhere in scale would be nice as well.

[mcorrell]

From #5 we now have "us" (which _de-_saturates colors as uncertainty increases) and "ul" (which lightens colors as uncertainty increases).