Change Model orientation convention to match the rest of Cesium

Fixes #1940 

As discussed in the linked issue, this change applies a rotation to all models behind the scenes so that the public `modelMatrix` property is consistent with the rest of Cesium.  After some help from @bagnell; I had to transform `BoundingSphere` center as well.  I also tweaked the Sandcastle Models example because they were zoomed in too closely after the recent fov changes.  This also fixes CZML model orientation, which have been broken for a while.

_NOTE: There are still 3 test failures in this branch that I'm having difficulty figuring out.  I opened this so that someone (maybe @bagnell or @pjcozzi) can take a quick look.  I'd appreciate it. The failures are all related the single-pixel rendering tests; the models themselves work fine (I tested them in Sandcastle just to be sure)_