Delete the history.

def clear(self):
        """Delete the history."""
        self.sampler.clear()
        self.samples_list = self._comm.gather(self.sampler.samples, root=0)
        if hasattr(self.sampler, 'weights'):
            self.weights_list = self._comm.gather(self.sampler.weights, root=0)
        else:
            self.weights_list = None