Equivalent numpy's nonzero but returns a tuple of Varibles.

def _nonzero(self):
        """ Equivalent numpy's nonzero but returns a tuple of Varibles. """
        # TODO we should replace dask's native nonzero
        # after https://github.com/dask/dask/issues/1076 is implemented.
        nonzeros = np.nonzero(self.data)
        return tuple(Variable((dim), nz) for nz, dim
                     in zip(nonzeros, self.dims))