This change is for a few reasons:

• it works with color, and (therefore) will need to be color managed, at

some point. This will be much easier to do if the code is closer to the
actual color management code (in Blender's core, so to speak).

• it has nothing to do with the actual fire simulation, as it is just

used to create a lookup table

• it can be reused for other purposes (i.e. in Blender internal

renderer, if people are interrested in a blackbody node à la Cycles)

• cleanup: some functions (contrain_rgb, xyz_to_rgb) already exist

in BLI