Add more fine-grained check levels

[nuest]

As of now, the check is binary. This requires the outputs to be pixel-perfect, whereas reality might require a human reviewer to make that call. Therefore we should discuss to loosen the binary nature of the check and...

• consider adding different result levels, better distinguishing
• consider not requiring images to be pixel perfect - can the checker distinguish between grayscale changes and completely different curves in a graph?
• consider allowing reviewers to override a result (transparently), maybe with a specific status check_overruled ?