Implement restart feature

[nicl-nno]

The restart feature can be useful during experiments:

1. The run of optimiser produces the files with full saved state
2. Then, the optimisation can be continued from the saved state if necessary.

Also, it makes it possible to restore session after software or hardware fails.

[nicl-nno]

https://machinelearningmastery.com/check-point-deep-learning-models-keras/

Can be useful as analogue