Allow users to set a default directory for data downloads to reduce strain on internet connections and speed up tests

[Robinlovelace]

I suggest that if a user sets an environment variable, e.g.:

STATS19_DOWNLOAD_DIRECTORY=~/data/stats19

the package uses that directory, not a random tempdir() as the place where it looks for and downloads data, speeding up testing and reducing bandwidth requirements for developers and researchers wanting to store the data.

This has been demonstrated successfully in the geofabric data package: ropensci/osmextract@a3c180c

[layik]

Reopen if not sufficient.

[Robinlovelace]

Great work @layik, many thanks!