Added demo code that was in BrainardLabPrivateToolbox.  It belongs here.

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

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

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