Enable "cache" behavior by default when downloading OSM data

[alronlam]

Hello, noticed that the OSM data download does not support any kind of caching.

It would be nice to support this natively so users don't have to keep writing their own file existence checks when they need to re-run cells in a Jupyter notebook or re-run scripts. Otherwise, the line of code would download the file again, resulting to long runtimes.

Maybe we can:

• Add an optional param like overwrite to the function. E.g. geofabrik.download_geofabrik_region("laos", "../test_dir", overwrite=False)
• Make this False by default, so that caching is enabled by default.
• This param will still allow users to overwrite the file should they wish to do so (e.g. it's been a while and they want to get a newer version of the OSM data).