Showing a problem with SceneCache SOP loading a PointsPrimitive.

The points are doubling up on each cook, because they aren't linked to any Houdini primitive.
Removing code that wasn't doing anything
Adding a ParticleSystem in ToHoudiniPointsConverter
PointsPrimitive Uniform prim vars are now converted.

Since we have a ParticleSystem for each PointsPrimitive, we can support conversion of the Uniform prim vars to Houdini PrimAttribs.
SceneCache tagFilter supports SceneInterfaces with no tags
Testing HoudiniScene with flattened geometry.

Adding tests showing that it doesn't support flattened geometry (all in one SOP) unless every level of the hierarchy has its own shape.
Testing SceneCache ROP for flattened geometry.

Adding test showing that it doesn't work for flattened geometry, because the HoudiniScene is returning the wrong results.
Fixed HoudiniScene bugs for flattened geometry.

Gaps in the hierarchy are now supported, and this commit fixes the tests from the previous two commits.
Merge pull request #93 from andrewkaufman/pointsInScc

SceneCache SOP loading PointsPrimitives
Merge pull request #94 from andrewkaufman/emptyTags

SceneCache OBJ tagFilter supports SceneInterfaces with no tags
Merge pull request #95 from andrewkaufman/houdiniSceneFlatGeoBug

SceneCache ROP flattened geometry bug