3D Tiles 1.0 Updates

These are the updates required to load tilesets defined by the 3D Tiles 1.0 spec. @ggetz if you think of others feel free to add to this list.

Now:

• Add support for b3dm RTC_CENTER #6488. The glTF 1.0 extension CESIUM_RTC will still be supported but this is the preferred method.
• content.url changed to content.uri. Support loading both, but if content.url is used log a deprecation warning. Once the PR is merged, add a note to #5363.
• Load batch table hierarchy extension. Still support loading the non-extension version, but log a deprecation warning. Once the PR is merged, add a note to #5363.
• Update all tilesets in Specs/Data/Cesium3DTiles with the upgraded tilesets in 3d-tiles-tools: #5504 and https://github.com/AnalyticalGraphicsInc/3d-tiles-tools/pull/139

Later:

• Add extras to API #6490. No need to add extensions to API yet.
• Update any 3D Tiles links in the documentation after the 1.0 branch is merged.
• Standardize the deprecation messages for anything in https://github.com/AnalyticalGraphicsInc/cesium/issues/5363.

[ggetz]

@hpinkos https://github.com/AnalyticalGraphicsInc/cesium/issues/5504

3d-tiles-tools has been updated to generate 1.0 tilesets, so we should be good to at least get rid of the sandcastle tilesets that throw deprecation warnings before the next release.

[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/forum/#!topic/cesium-dev/8zJhev5Vh5w

If this issue affects any of these threads, please post a comment like the following:

The issue at https://github.com/AnalyticalGraphicsInc/cesium/issues/6697 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

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