- adds consistency with regular point-to-point parallel streams.

ENH: add ITstream::tokens() methods

- useful, for example, to disambiguate constructing tokenList from
  a ITstream.

  ITstream& is = otherDict.lookup("original");

  tokenList deepCopy(is);  // Error: from Istream or tokenList??
  tokenList deepCopy(is.tokens());  // OK: from tokenList