use polite scraping

[brry]

Consider using polite for downloading files. Maybe this is already conceptually fine with no re-downloads and CURL handle in https://github.com/brry/rdwd/blob/master/R/indexFTP.R#L121

[brry]

indexFTP might be fine, but dataDWD uses download.file with no checks. When calling dataDWD with, say, 500 urls, once banned, it will still keep trying the rest of the urls. Definitely not polite.