[coneInc] = DKLToConeInc(dkl,bg)
Convert from DKL to incremental cone coordinates.
The DKL coordinate system convention is (Lum, RG, S)
The code follows that published by Brainard
as an appendix to Human Color Vision by Kaiser
8/30/96 dhb Converted this from script.
10/5/12 dhb Comment specifying coordinate system convention.