make fromHSL public, add toHSL

[Olaf Schmidt]

[ Matthew Woehlke, Mi., 13. Jun. 2007 ]
> IMO your color space is correctly termed H'C'Y' :-)

I have never heard of H'C'Y'. Earlier you called it H'S'Y', which is a group 
of colour spaces.

Since we are taking about "correct" terminology:

sRGB and Adobe RGB are colour spaces. RGB is a group of colour spaces which 
have a red, green and blue component. R'G'B' is a group of gamma-corrected 
RGB colour spaces.

HSL is a group of colour spaces with hue, saturation and luminosity 
components, where L=0 means black, L=MAX means white, V=0 means gray, and 
V=MAX means full saturation. H'S'L' is a group of gamma-corrected HSL colour 
spaces. For some of the H'S'L' colour spaces, the luminosity component is 
hue-independent (which implies gamma-correction). This seems to be what you 
call H'S'Y' or H'C'Y'. One example for such a colour space is sHSL (derived 
from sRGB).

Olaf