arxivUrl( arxivId )
If the supplied string appears to be an arXiv identifier,
it strips any "arXiv:
prefix
and prepends the string "https://arxiv.org/abs/
".
arxivId
(String)arxivUrl("arXiv:1804.09379") =
"https://arxiv.org/abs/1804.09381"
String arxivUrl(String)