Increment weights: lw <-lw + delta. 

        Parameters
        ----------
        delta: (N,) array
            incremental log-weights

def add(self, delta):
        """Increment weights: lw <-lw + delta. 

        Parameters
        ----------
        delta: (N,) array
            incremental log-weights

        """
        if self.lw is None:
            return Weights(lw=delta)
        else:
            return Weights(lw=self.lw + delta)