OTF restart module/method

[YuuuXie]

Occasionally OTF could be interrupted (either by bug, or the user wants to interrupt the training and change the condition) and needs to restart from middle. It will be good to have a restart module, or a method inside otf module.
I have a script to do this while I was training my stanene system. If you guys have written any better wrapped module, that will be great.

[stevetorr]

I plan to add this as a flag into the gp_from_aimd.py, it should be a simple check to write the model by serialization every few steps.

[YuuuXie]

close via #214