Improvements to mesh graphics #959

Once #953 is merged we'll have general mesh, surface, and polygon graphics.

• cmap support with HistogramLUTTool It makes sense to have colormaps for mesh and surface that are similar to image colormaps.
• mapcoords as a feature, maybe the same feature/similar to cmap_transform, see: Add support for mesh and surface #953 (comment)
• heat diffusion example?
• tests