Added basic support for RenderMan volume shaders.

The type of a RenderMan volume shader itself is simply "ri:volume", but that's not a valid type for actually assigning it to an object - we must use either "ri:atmosphere", "ri:interior" or "ri:exterior". This patch ensures that by default a volume shader will be assigned as "ri:atmosphere", but allows the type to be changed following loading. The type can be changed as follows :

`script["myShader"]["type"].setValue( "ri:interior" )`

We'll need some UI for setting this at some point - for the moment this pull request is just the bare minimum to get Beau working, and to get me on to other priorities...