Allow for the spaghetti option to plot GAM smooths as well

[AdamCSmithCWS]

probably best to add a gam = FALSE argument to plot_indices, same as what's used in the generate_trends function

[AdamCSmithCWS]

e.g.,

inds <- generate_indices(fit,hpdi = TRUE, gam_smooths = TRUE)

tmp <- inds$gam_smooth_samples$continent_continent %>%  
as_tibble() %>% 
slice_sample(n = 100) %>% 
mutate(iteration = 1:100) %>% 
pivot_longer(cols = -iteration, 
   values_to = "index", 
   names_to = "year")

tst <- ggplot(data = tmp)+
geom_line(aes(x = year,y = index,group = iteration),
                  alpha = 0.3)+
scale_y_continuous(limits = c(0,NA))+
theme_bw()