Ordered Dictionary of lists with the evolution of
        the data as function of the relaxation step.

def history(self):
        """
        Ordered Dictionary of lists with the evolution of
        the data as function of the relaxation step.
        """
        history = OrderedDict()
        for cycle in self:
            d = cycle.last_iteration
            for k, v in d.items():
                if k in history:
                    history[k].append(v)
                else:
                    history[k] = [v]

        # Convert to numpy arrays.
        for k, v in history.items():
            history[k] = np.array(v)

        return history