ENH: enabling color image support

Number of components now set from image message.
Display node color table now set depending on number of components.

I'm not sure if there's a clean way of downcasting an igtl::MessageBase into a igtl::ImageMessage so for now I operate on the inner pointer. If anything fails it reverts to the original greyscale image.
ENH: enabling color image support

Number of components now set from image message.
Display node color table now set depending on number of components.
Repariring whitespace
Cleaning up the message base to prevent programmer error and to simplify future development
STYLE: Fix indent style.