Houdini Object Converters

This request adds converters between Objects and GU_CortexPrimitives. Doing so required changing the base converters to deal with Objects rather than VisibleRenderables, which breaks binary compatibility in IECoreHoudini. I've also added special converters for CompoundObjects, so the children will be exposed seperately inside Houdini, and re-collected into a Compound on the way out.

Now that GU_CortexPrimitives can be handled with converters, I've updated the generic File SOP, HoudiniScene interface, and SceneCache SOP to use them when appropriate. SceneCache SOP now also avoids copying the object read from disk whenever possible.