ENH: provide refOrNull method for autoPtr.

- Normally use '()' to deference. This has extra safety and issues a
  fatal error if the underlying pointer is not valid.

  However, in some cases we are happy with getting a null reference.
  The refOrNull() method returns the reference without any checking.

  Usage example:

      autoPtr<OFstream> osPtr;
      if (Pstream::master())
      {
          osPtr.reset(new OFstream(...));
      }
      writeViaMaster(osPtr.refOrNull());

      - The writeViaMaster() call takes an OFstream reference,
        but this is only used directly on the master.
        The slaves will pass things through to the master.