Implements #57: Add support for editing Level 6 metadata blocks
Edited when already existing, created when not.
Add support for madVR flags=3 measurement files, to use target nits per frame.
Use the --use-custom-targets flag when generating.
- The target nits are used as L1 peak nits per frame, but they are not as smooth as per-shot metadata.
- The scene cuts and average nits are dynamic per-shot.