Resolves #69. Added 2D transfer function support and updated Volume object type by removing subtype as it really isn't needed similar to how subtype is not needed for the Surface object type.
Also added a feature to the Renderer for explicitly specifying the volume rendering method.
Working group decided (4/26/2023) to stick with the current design but change the subtype name to better reflect the type of volume and avoid confusion and perceived connection to Renderer subtypes.
Resolves #69. Added 2D transfer function support and updated Volume object type by removing subtype as it really isn't needed similar to how subtype is not needed for the Surface object type.
Also added a feature to the Renderer for explicitly specifying the volume rendering method.