Support Partial output for Operations

[jnation3406]

Some operations which operate on input images sequentially (Normalize, Subtraction) could report back output images as they are created. This would provide a better user experience if the user is sitting there and waiting for the results. I think it might be as simple as just updating the output array periodically during the operation, and then modifying the frontend code to check for output updates during IN_PROGRESS and not just during COMPLETED.