ENH: add tmp/refPtr support for setting cref/ref from pointer

- makes it easier to use for local or alternative storage.
  Eg,

  ```
      tmp<volScalarField> tfld;

      tfld.cref(obj.cfindObject<volScalarField>("name"));

      if (!tfld)
      {
          tfld = volScalarField::New("name", ...);
      }
  ```