made LinkedScene( SceneInterfacePtr ) support writable scenes
Fixed SceneCache from IndexedIO bug, added a test
SceneCache SOP transforms all Point, Normal, and Vector primitive variables.

Previously it was only transforming P and N.
Merge pull request #193 from davidsminor/linkedSceneFromIndexedIO

LinkedScene can now wrap writable SceneCaches
Merge pull request #192 from andrewkaufman/sccSopPrimVarTransform

SceneCache SOP transforms all Point and Normal primitive variables.