SceneCache Rop crash and speed issues

This request address issues with the SceneCache ROP in Houdini. The first commit fixes a crash for caching hidden OBJs. The rest of the commits deal with the speed of caching. A large production asset (500,000 polys with 1800 named shapes in a single SOP) was taking over an hour per frame to cache. Following these changes, the same asset takes 3.3 seconds per frame. This means that a 200 frame cache was taking over a week, and now takes 12 minutes.

One of the optimizations, which may be useful elsewhere, is the addition of the DetailSplitter class. This is used to take a GU_Detail and split it by name in an efficient manner. The first call to split( name ) will actually create all named variations, speeding up any subsequent requests.