Authentication lookup helper

get_clowderauth(key, user, pass, url, authfile = "~/.pecan.clowder.xml")

Arguments

key, user, pass	passed unchanged from `download.Geostreams` call, possibly null
url	matched against `<hostname>` in authfile, ignored if authfile contains no hostname.
authfile	path to a PEcAn-formatted XML settings file; must contain a `<clowder>` key