Diffusion visualization not quantitative: no scalar bar

[ihnorton]

[https://issues.slicer.org/view.php?id=3859]

Due to the lack of a scalar bar option, it is not possible to comprehend the colormap. What FA values are displayed along a tract?This is a problem for generating images for publications using Slicer. It is also a general issue when trying to visualize diffusion data.See screenshot. It is not clear in any way how the colors map to numbers, or even what color is max or min.

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