allow custom comparison plots

This extends !98 (merged) and allows comparison plots to be generated.

I changed the search for __all__ to __single_plots__ for plotting single results and __comparison_plots__ for plotting many, although I guess this could be averted by requiring that all the functions can plot either single results or many results.

I also added a bunch of plt.close() statements after plots had failed as there were figures being opened and not closed.

@charlie.hoy I'm happy to make any style changes you want.