Dump nep restart periodically

[elindgren]

Summary

Adds a parameter to save_potential to optionally write nep_genX.restart when writing nep_genX.txt during training.

Modification

• Add a parameter to save_potential, called save_potential_restart.
• Write a function to generate the nep_genX label.
• When writing the regular nep.restart file, check if a nep_genX.restart file also should be written.
• SNES::output_mu_and_sigma now takes a second keyword argument, which is the filename for the restart file to be written.

Others

🐈‍⬛

[elindgren]

ping @erhart1

[erhart1]

ping @erhart1

🥳

[brucefan1983]

looks good to me