Move warning because of large fit residuals to JSON

[giovannipizzi]

Also:

• check if the current threshold is a good one
• display that there are high residuals in the plots (e.g. dashed line, or similar other approach?)

[giovannipizzi]

Now the residuals are in the JSON, the plots are coloured in orange if the residuals are too hugh, and the value has been tuned to a reasonable value