Material authoring workflow

[nrusch]

We need a workflow for authoring Hydra materials in Nuke.

The most brute-force version of this seems like it might be a generic SdrShader op which can represent any Sdr shaing node type (similar to the renderer-specific shading node concept in Katana). The available node types can be discovered at runtime, so we can build menu items for them.

This will require some further development and hardening of the KnobFactory mixin.