Define a more flexible convergence criteria

[JoelPasvolsky]

Current implementation sets as a convergence criterion that the resulting energy is unchanged through convergence number of iterations. In practice that will be infrequently triggered because of fluctuations in the energy landscape. It would be helpful to define a more flexible criterion such as a percentage of energy decrease in a window of iterations. So setting, for example, convergence_x=10, convergence_y=5 would terminate if the energy has decreased >= 5% from its value 10 iterations ago. ("x", "y" could be more intuitive names.) Thank you!