For internal use only, returns the URL to the .grb file given
the input parameters. Called by the download functions.
eupp_get_source_urls(x, fileext = NULL, ...)object of class eupp_config returned by eupp_config.
NULL or character length 1. File extension (or
URL extension) added to the URL. Used to generate URLs to GRIB index files.
Defaults to NULL.
unused.
Returns a character of length 1, URL to retrieve the data set.