Wrong orientation of 3d tiles dataset

[jasonbeverage]

The attached 3d tiles dataset is oriented incorrectly, like it's already z-up when it's supposed to be y-up. Taking out the y-up to z-up transform in makeNodeFromModel makes it appear correctly. Oddly enough it is oriented correctly in Cesium so there is some difference there.

See forum discussion at http://forum.osgearth.org/ThreeDTilesLayer-issue-td7593305.html#a7593308
shape.zip

[jasonbeverage]

The tileset.json has an attribute on the asset element like this:"gltfUpAxis": "Z".
This isn't part of the 3d tiles spec, but it looks like at one point it was supported: CesiumGS/cesium#5005.
Cesium still supports it apparently in their code, so perhaps we should support it even though it's not commonly used. Maybe we can just pass in an option to the gltf loader indicating whether or not to do the y to z up transform.