make vignette more compact

[njtierney]

Currently it takes a long time to fit - would be good to resolve this using smaller data and examples

[njtierney]

use downsampled data like so:

contact_data <- get_polymod_contact_data("all")  %>% 
  filter(age_from <= 10,
         age_to <= 10)

population <- get_polymod_population() %>% 
  filter(lower.age.limit <= 10)

[njtierney]

Perhaps wrap these in a function like "get_example_data_*"