Barb Renderer: Auto-color by wind speed

[sgpearse]

The Barb Renderer seemingly shortens and lengthens wind barbs according to the magnitude of its field variables. 99% of the time the length represents wind speed, and this means that we are already calculating speed without forcing the user to interact with the Python Engine.

A much better user experience would be for our Color Mapped Variable to default to the "Field Magnitude" that we are already computing.