Export `set_data_directory()` and document it in the README

[agila5]

I think set_data_directory() is an extremely useful function to set a persistent directory for stats19, but it's not exported:

remotes::install_github("ropensci/stats19", quiet = TRUE)
library(stats19)
#> Data provided under OGL v3.0. Cite the source and link to:
#> www.nationalarchives.gov.uk/doc/open-government-licence/version/3/
stats19::set_data_directory
#> Error: 'set_data_directory' is not an exported object from 'namespace:stats19'

^{Created on 2021-10-20 by the reprex package (v2.0.1)}

I think we could 1) export the function and 2) add an example in the README. I can add a PR if you want.

[Robinlovelace]

I can add a PR if you want.

Big +1 to that idea!

[Robinlovelace]

After #210 is fixed?