Handle multiple forms of color

[bsaund]

I want to handle the color cases:

1. No color is provided and the color is set by the rviz plugin
2. A single color is provided by the message, which the visualizer can choose to use
3. A color per voxel is provided, which the visualizer can choose to use

I suspect a good approach would be to copy the style of the DepthCloud Display, where there is a selectable "Color Transformer". The RViz user could choose and set "flat color" (option 1), "Use message monocolor" (option 2) or "Use color per voxel" (option 3). If the color fields of the message is not set, only option 1 would be reasonable.