Houdini Attribute Copy for SceneCaches

This request adds an attributeCopy parameter, which can be used to duplicate primitive variables inside the SceneCache readers, before converting them into Houdini. It uses space,colon separated syntax such as "a:c b:d". The most common use of this parameter in the IE pipeline will be to copy P to Pref (which then gets auto-translated to rest on native Houdini geo).

As part of this, I have reorganzied the parameters on the SceneCache OBJs, moving the various geometry options and filters to a new Options tab. As such, the first commit below is taken from my transformOverride pull request #166 which would otherwise prevent this from merging.