HUC4 subregion are specified by a unique character string, best obtained using the get_huc4
function.
download_nhd_subregion
returns the path to the downloaded zip file.
download_nhd_subregion(area, raw.dir)
A 4-character string indicating the HUC4 NHD subregion to download.
A character string indicating where raw downloaded files should be put. The directory will be created if missing.
A character string representing the full local path of the downloaded zip file.