Add schematic of coordinate system

[cpchuang]

Please add a schematic of coordinate system used in HEXRD in "Help->coordinate system" so users can understand the orientation of each axes.

[psavery]

We can definitely do this. I mentioned this to @saransh13, and he agreed that the z-direction is non-intuitive, but it was chosen a long time ago and would be hard to fix at this point. A schematic of the coordinate scheme will definitely be helpful.

[psavery]

I think we can use Figure 1 from this document for this!