Feature/3d scale indicator

[frasercl]

Add tick marks to the bounding box as in Simularium, along with Simularium's 2d indicator of spacing between tick marks.

The spacing between tick marks is always some power of 10 physical units that produces at least 2 tick marks along the volume's longest axis. The scale indicator matches the color of the bounding box, and is only visible when all these conditions are met: the viewer is in 3d mode and not path tracing, the bounding box is enabled, and scale bars have not been disabled with View3d.setShowScaleBar.

Combined with #82, this resolves #66.

Screenshots

(From Cell Feature Explorer)

[toloudis]

This is looking awesome and will solve a real "quantitative" use case for our users.

First impressions from the screen shots:

• tick marks are too long, half size might be fine?
• 10 tick marks seems like too many. Smaller tickmarks may simply be enough to solve that and make it look less intrusive. Otherwise we might want to have a dynamic scale value or get one more UX intervention. By dynamic scale value, find an integer number of units where the number of tick marks on the longest axis will be 8 (for example).

[frasercl]

Will be worth circling back with UX team later about number of tick marks, among many other small questions, but I'm comfortable with the state this is in now.