Add a specific plot method for predictions

[zsusswein]

In #76 @seabbs wrote:

I think we should have a issue for making these low level ggplot calls into their own standalone functions that take the output of predict as an alternative user path

My read is that this would be something like:

fit <- RtGam(stochastic_sir_rt["obs_incidence"]], stochastic_sir_rt[["date"]])
preds <- predict(fit) # Maybe I want to apply some funky options here
sums <- summary(fit) # Or maybe I want to use the summary method

# And I can have some standardized plotting method
plot_predict(preds)
plot_predict(sums)

[seabbs]

yes exactly or alternatively

plot_rt, plot_obs etc and design those to take the output of predict

or yet more s3 I suppose