Add support for Region Bounding Volume

[gkjohnson]

Regions should be specified in the local frame of the group treating the group as the WGS84 coordinate frame.

Example data here.

Spec here.

More information in CesiumGS/3d-tiles#445

[gkjohnson]

Add support for:

• A Region class
• A RegionHelper class for rendering a region
• A displayBoundingRegion flag for debug tiles renderer

The region must be defined on the oblate WGS84 spheroid with major and minor axes defined here