If data.dir=NA, the data storage directory is set to the a subdirectory of base::tempdir(),
a per-session temporary directory (cleared after each R session). This happens automatically if
opendata_bc or getdata_bc are called before datadir_bc, ensuring that rasterbc won't
unexpectedly overwrite things or leave garbage in the user's file system.
However, users are strongly encouraged to set the data directory manually to a non-temporary
location. This allows copies of downloaded data to persist between sessions, so that
rasterbc::opendata_bc can load the local copies in future R sessions. This is both much faster
than downloading mapsheets repeatedly, and it reduces the strain on FRDR's data hosting service.
The directory data.dir will be created if it doesn't already exist.