Add routines for converting between wavelength of light and frequency of light.

Signed-off-by: David Brainard <brainard@psych.upenn.edu>
Code to handle pigment bleaching in FillInPhotoreceptors, plus added routine to compute fraction pigment bleached.

Signed-off-by: David Brainard <brainard@psych.upenn.edu>
Modifications to allow computation of effects of pigment bleaching on cone spectral sensitivities.

I think this all works OK, but will take a pass over next few days to clean up the design a little, as well as better comments etc.

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