Standardize default Entity orientation.

1. Added a new private (for now) function, `Entity._getModelMatrix` to centralize creating a modelMatrix from an entity instance. This function is the same as the `ModelVisualizer` behavior, which defaults to ENU when no orientation is provided.
2. Refactored `ModelVisualizer`, `EllipsoidGraphics`, and `CylinderGraphics` to all use `Entity._getModelMatrix`.