Test and validate B3DM Batch Table and Feature Table support

[gkjohnson]

See example files here.

[gkjohnson]

Feature Table

Describes general pieces of data or "features" within a model, such as a building:

https://github.com/CesiumGS/3d-tiles/tree/master/specification/TileFormats/FeatureTable

Every model / feature / vertex may get a batch id to associate it with information in the batch table.

Batch Tables

Per-batch id information -- batch ids are associated with different features.

https://github.com/CesiumGS/3d-tiles/tree/master/specification/TileFormats/BatchTable

B3DM

Feature Table

Describes the number of batch ids in the batch table.

Batch length can be derived from BATCH_LENGTH feature value.

I3DM

Feature Table

Describes the number of instances.

Batch length can be derived from BATCH_ID array if it exists.

PNTS

Feature Table

Describes the points.

Batch length can be derived from BATCH_LENGTH feature value.

[gkjohnson]

Implemented and verified in #81.

Closing in favor of #65, #68