Add functionality for manipulating NEP restart files

In order to fine tune models (e.g., based on NEP89), calorine should provide functionality for reading, modifying, and writing nep.restart files.

Todos

• read and write nep.restart
• modify restart information
  • remove species
  • clear/convenient way for setting means and variances; separately for descriptor, network, and possibly by species
• synchronize with reading/writing nep.txt (needed when removing species)