What problem does your feature request solve?

Q-Q plots are traditionally done for comparing the same diagnostic in multiple models/models and observations. Originally the Q-Q functionality in CSET was a basic (different variables in the same model) as multiple models could not be loaded at the time. Given this multiple model loading functionality exists we should add this functionality into CSET so that we can produce sensible Q-Q plots. This will also need to be implemented with a new loader.