Use temporary directory by default when downloading data

[Robinlovelace]

CRAN can be strict on writing to the hardrive, e.g.: ropensci/osmextract#175

I've not checked but in case some of your functions write to HD by default, I think it's worth making it use tempdir() by default.

This is how we tackled it in {osmextract} and in the prototype {spanishoddata}:

get_data_dir = function() {
  data_dir_env = Sys.getenv("SPANISHOD_DATA_DIR")
  if (data_dir_env == "") {
    data_dir_env = tempdir()
  }
  return(data_dir_env)
}

Source: https://github.com/Robinlovelace/spanishoddata/blob/73528cb2e867c9e99456cbe67fe1c6b08bca8dc8/R/get.R#L63C1-L69C2

And the more mature {osmextract}:

You can set a persistent download_directory by adding the following to your .Renviron file (e.g. with edit_r_environ function in usethis package): OSMEXT_DOWNLOAD_DIRECTORY=/path/to/osm/data.

Source: https://docs.ropensci.org/osmextract/reference/oe_get.html

Happy to have a go at this one at some point...