Compute auxiliary weights (for APF).

def setup_auxiliary_weights(self):
        """Compute auxiliary weights (for APF). 
        """
        if self.fk.isAPF:
            self.logetat = self.fk.logeta(self.t - 1, self.X)
            self.aux = self.wgts.add(self.logetat)
        else:
            self.aux = self.wgts