Add support for Babylon tiles renderer

[gkjohnson]

Fix #387 Related to #1373 Related to #1375 Related to #1390

• Move BabylonJS tiles renderer implementation from this repo.
• Export renderer as 3d-tiles-renderer/babylonjs.
• Add demos, basic docs
• Reduce inline math instance creation

TODO

• Use babylon gui system?
• Add attributions to demos
  • requires accessing gltf "asset" meta data fields
• Use Babylon math functions when available
  • OBB, Region, Ellipsoid
  • Plane "transformToRef"
• ~Use the correct GLTF Loading system~
• ~Use correct imports~
• ~Add "events" support (dispatch? Other "babylonjs-esque" approach like observers?)~

Open Questions

• Support left handed coordinate system
• Bytes used function
• Cache more logic in "update" loop

[gkjohnson]

cc @georginahalpern

[gkjohnson]

@georginahalpern - thanks for the comments. All of the suggestions sound good to me so I'm happy to take PRs for them. I don't want to merge or publish babylon support before it's ready so if it's okay lets consider this babylon-support branch the babylon dev branch and make any PRs in it. Then once some of the big TODOs are addressed / answered (math class support, correct imports, etc) we can merge into master and publish.

[georginahalpern]

@gkjohnson first PR with some minor changes https://github.com/NASA-AMMOS/3DTilesRendererJS/pull/1397