Implement the CIEDE2000 color difference formula for CIE L*a*b*

[Ogeon]

See https://en.wikipedia.org/wiki/Color_difference for reference. The formula is a bit complicated, to compensate for a number of shortcomings in the L*a*b* color space, but seem to be the currently best measure for color difference in L*a*b*. We should spare the users the effort of implementing it themselves.