Add COPs support

[TomMinor]

Changes proposed in this pull request

• In the material translation we need to detect op: paths and do something about it. If an op: path is detected, I create the original image node as normal but also create a HtoA image_cop node with the original image node as reference, so all the image attributes can be copied automatically. That way all of the Houdini library pain is avoided by arnold-usd and the Houdini business of dumping the pixel data is done for me. This should work for any Houdini version.
• As explained in the HDK Hydra docs, we need to check for the houdini:cop_texture_changed render setting and update our texture cache. Flushing the universe texture cache does the trick for now but I would like to investigate the performance implications of this. This is only supported in Houdini 21+

Additional context