Add some missing private methods in PropertyTable for batch table hierarchy

[ptrgags]

There were a couple private methods of PropertyTable (and ModelFeatureTable) that were missing concerning the batch table hierarchy extension:

• isClass()
• isExactClass()
• getExactClassName()

This fixes the 3D Tiles Batch Table Hierarchy when enableModelExperimental = true

Sandcastle for testing

@j9liu could you review?

[cesium-concierge]

Thanks for the pull request @ptrgags!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[j9liu]

@ptrgags looks good to me. The sandcastle works for me now, let me just run local tests to confirm they work and then I'll merge. Thanks!