Rewrote to use staircase to determine test levels.  This should decrease the likelihood of pathological choice of test levels.

Also decided not to enforce gamut limits on test levels.  Might as well see how far you need to go to reach threshold.  Can always interpret data relative to gamut at the time when we compute and fit the contours.

Signed-off-by: David Brainard <brainard@psych.upenn.edu>