[Radiance-general] Yxy to XYZ

[Randolph M. Fritz]

For the record (since I had to work this out) the formulas are:

X = x * Y / y
Y = Y (of course)
Z = -(Y * (x + y -1)) / y

Wolfram Alpha can solve equations, which was really useful when I was 
tired and didn't want to fiddle with the algebra.
-- 
Randolph M. Fritz • RFritz at lbl.gov
Environmental Energy Technologies Division • Lawrence Berkeley Labs