Clean up of GetHumanPhotopigmentSS

a) Add comments and simplify return interface.
b) Fix buglet where melanopsin shift was 1 nm not 2 nm.
d) Fix up test program for new interface.
d) General cleaning of test programs in this directory.
e) Test plots go in subdirectory.

Signed-off-by: David Brainard <brainard@psych.upenn.edu>
Small change to comments in here.  Awaiting more detail from MS to fill it out better.

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