Adding thread-safe attribute read to SceneCache

This problem is mainly detected when the root location of scene caches have attributes. The root scene caches are returned by our SharedSceneCache mechanism and therefore parallel threads could be querying at same time.
Added a unit test to demonstrate that.
Merge pull request #263 from ldmoser/threadSafeAttributeReadSceneCache

Adding thread-safe attribute read to SceneCache
Working around a bug in Maya for meshes with 6 or more UV sets.

When calling fnMesh.numFaceVertices() (and other MFnMesh API calls), given a mesh with 6 or more UV sets, which has never been evaluated before, the first call throws kFailure. From within the ToMayaMeshConverter itself, the output plug appears fine, and the API calls evaluate as expected. Despite this, the resulting mesh cannot be evaluated on the first try. Making the mesh visible, or making any attempt to evaluate it, will trigger some unknown internal updating, and subsequent attempts to evaluate it will succeed. Meshes with 5 or less UV sets do not suffer from this problem.

I've added a test to FromMayaMeshConverter which demonstrates how this issue can cause a seg fault, and I've patched FromMayaMeshConverter with a silly fix, so the problem does not occur. Unfortunately, I could find no solution that solved the overall problem, and I've added a test to ToMayaMeshConverter which demonstrates the issue.
Fixing precision issue.
Merge pull request #264 from andrewkaufman/meshConvertCrash

Mesh converter crash with many UV sets
Fixed a bug in the SceneCacheReader that was preventing animated scene caches from displaying properly.