Continuing to clean up and extend isomerization calcs.

Most of this effort is just checks and commenting.  It is possible that some of the checks will break calling code that was a little sloppy.  I believe I've fixed up all such cases within the PTB.

Added AbsorbtanceToAbsorbance, from the Stanford group.

The isomerization calcs now more cleanly incorporate the CIE standard cone fundamentals (that is, you can get them from DefaultPhotoreceptors).

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