Standard Entity default orientation support.

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`.
Merge remote-tracking branch 'origin/master' into default-orientation
Merge pull request #2355 from AnalyticalGraphicsInc/default-orientation

Standardize default Entity orientation.