Convert to S7

[thomasp85]

There are a bunch of user grievances from the dispatch constraints of S3. This makes patchwork a great candidate for kicking the tyres of S7

[thomasp85]

Initial tests reveal that the double dispatch works, but only by overwriting the + operator (something patchwork shouldn't do)

Since patchwork doesn't own the classes it operates on it has to wait for ggplot2 to update to S7