More error metrics

[singraber]

Improve error reporting capabilities. Previously only the RMSE of per-atom-energies and forces was computed and reported. Now the following quantities are taken into account:

• RMSE of energies per atom
• RMSE of energies
• MAE of energies per atom
• MAE of energies
• RMSE of forces
• MAE of forces

All quantities will be written to learning-curve.out upon training. The output of nnp-dataset was also restructured to show all error metrics.

[singraber]

Also, added new keyword main_error_metric which controls the screen output.