should autoplot always return a ggplot-object?

[mb706]

it appears that autoplot() for OptimInstanceSingleCrit with default args does not really return a ggplot-like object. One cannot do

autoplot(instance) + theme(legend.position = "none")

for example. Instead, a DelayedPatchworkPlot is returned, which causes patchwork::wrap_plots() to be called for plotting. We should probably try to avoid this if possible, since being able to modify autoplot()-plots post-hoc would be very useful.