lab = XYZToLab(xyz,whiteXYZ)
xyz is a 3 x N matrix with xyz in columns
whiteXYZ is a length 3 vector with the white point.
lab is a 3 x N matrix of [Lstar;astar;bstar]
Formulae are taken from Wyszecki and Stiles, page 167.
xx/xx/xx baw Created.
xx/xx/xx dhb Made compatible with version 3.5
10/10/93 dhb Changed name to XYZToLab.
5/9/02 dhb Improved help