Sanitize "Core/Color.hpp".
Replace ChannelTraits class by two simpler functions.
Remove unused typedefs.

Seems useless to me.
Fix OpenGL-like typedefs.
Fix primary color functions.
More explicit comment inside function 'int_rescaled_channel'.
Replace deprecated 'Color' by 'Pixel' class.
Remove unused and untested function.
Reworded docstrings in 'Image' class.
Fix color conversion functor 'ConvertImage'.