Adding HoudiniScene::readWorldTransform and readWorldTransformAsMatrix

These can be used to get the world transform at the specified location at the correct time.
Avoiding seg fault when attribute data is null
Adding wrapper class for SceneCacheNodes and binding scene() method
Using IECore::msg rather than cerr
Fixing issues in HoudiniScene when pointing to broken SOPs
adding todo
Merge pull request #163 from andrewkaufman/brokenSopBug

Fixing issues in HoudiniScene when pointing to broken SOPs
Merge pull request #161 from andrewkaufman/houdiniSceneWorldMatrix

HoudiniScene world matrix
Replacing setNode/getNode with sceneNode
Merge pull request #162 from andrewkaufman/sceneNodeInterfaceBinding

Adding wrapper class for SceneCacheNodes and binding scene() method