BUG: still require IStringStream for argList (closes #672)

- regression introduced by part of commit e77edcf9.

  Specifically the way that scalarRanges is written, it parses through
  until it hits invalid input. This works fine with an IStringStream,
  but the ITstream is pickier and reports this as being an error.

  So revert to IStringStream and be less picky about argList parsing.
  in the longer-term, should fix scalarRanges instead.