#225 Add ruler and colorbar

[Martin Slavík]

I enjoy two visualization features present e.g. in VMD visualization extensions.
1) ruler (abscissa) with a number and unit that reflects present state,
2) color bar mapping used colors to numbers.

Ruler request is dated back to 2004, and although could be done with scripting, it is cumbersome. Positioning could be: top|bottom|right|left and style as a line, grid or tape. Example is attached.

I assigned this high priority because this is essential for proper understanding of model.