This MR, in tandem with SMTK!703, 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.