Houdini Light Converters

[andrewkaufman]

We'd like to be able to convert IECore::Lights to and from Houdini. Once we have this, we could start storing light rigs in SceneCache files.

[johnhaddon]

This one seems like it needs plenty of thought to me. We need to decide whether or not there are common properties of lights we can represent across all packages, and how we go about representing non-common properties. We also need to decide whether lights are objects in their own right or something more like cameras or coordinate systems that happen to have light shaders assigned. The IERendering approach has been that lights are first class objects, and that's the Gaffer approach too so far, but there are a few things that need working out about that still.