Fixing SceneInterface Time in Houdini

We need to adjust the time we read SceneCaches inside Houdini because Houdini treats time starting at Frame 1, while SceneInterfaces (and Maya) treat time starting at Frame 0. The SceneCache readers and writer (OBJs, SOP, and ROP) now translate Houdini evaluation time to SceneInterface time before querying any SceneInterface. As HoudiniScene is a SceneInterface, its methods now expect SceneInterface time, and translate back to Houdini time when querying Houdini. The ROP also forces time re-mapping when writing links, to account for the difference.