Add a glyph-mapped representation for instances.

This MR, in tandem with smtk!703 (merged), adds support for model instance entities. Instances are entities whose shape is dictated by a prototype model entity. The prototype's geometry may be transformed (via translation, rotation, and scaling) but is otherwise unchanged. This MR only supports translation, not rotation and scaling. A single instance may have multiple copies of its prototype. Each copy is called a placement.

In order to render instances, SMTK's source VTK filter provides additional outputs for prototype and instance-placement points. CMB exposes these via a new representation maintained by ModelBuilder.