Add Q-Q plot on normal distribution to functional testing

[MichaelClerx]

More intuitive than a KL divergence plot?
But harder to one versus time 😅

pints-team/pints#1239

[MichaelClerx]

However, we might consider having a set of "versus time" plots to see changes, and then a 2nd set of "last run" plots (or "last N runs") plots, showing plots useful for debugging / checking behaviour

[MichaelClerx]

Example of a diagnostic plot