SceneCache SOP loading PointsPrimitives

There is a bug in the SceneCache SOP when loading PointsPrimitives, whereby it will double up the points on every frame. This happens because the hierarchy traversal (and geo cleanup) is based on Houdini prims. At present, PointsPrimitives are represented as free floating points in Houdini, not associated with any prims.

This request makes the ToHoudiniPointsConverter create a Houdini ParticleSystem for each PointsPrimitives. The particle prim can be named and managed like any other Houdini prim (polys, curves, etc), and therefor allows the SceneCache SOP to update the associated points appropriately.