Replaced `SMALL_COLOUR` legacy type with new type `PhotonColour` based on new class template `GenericRGBEColour`; also employing this new type in Radiance HDR image file reading & writing.
More colour data type refactoring.

Now using different dedicated colour data types for:
- parsing (legacy RGBFT format)
- front-end result image processing (RGBT format; the front-end never actually used the filter channel)
- render engine colour math (currently RGBFT format; intended to be later replaced with RGB colour + RGB transmit)