#40 RGB display magnitude option for vectors

[Philip Cook]

In SNAP 3.2, the RGB value for vector data appears to depend on the sign of the vector components.

In DTI, typically we map (R,G,B) = (|x|, |y|, |z|).

There might be applications where the signed version is useful - one downside of the traditional RGB display is degeneracy between, say, (0.5, 0.25, 0.25) and (0.5, -0.25, 0.25). But sometimes the magnitude is easier to interpret.