Optionally visualise corner plots in notebooks

[rhayes777]

When an optimisation runs in a Jupyter notebook visualisations are output. Corner plots from the optimiser are saved to file but not visible in the notebook. Show these corner plots once in the notebook at least once the optimisation completes.

[Jammy2211]

The Analysis class should have an optional method, analogous to visualize, which allows a user to extend and define their own notebook output.

For example, in autolens, this would show the corner plot, the model.result text and the lensing speicifc subplot_fit.png.

I don't think cornerplots are very useful for "on-the-fly" updates, but there will be a similar plot which shows something like how the likelihoods are increasing as a function of iteration number. But getting it working for a cornerplot means I can swap it out from there.