[Feature Request] Append to model_devi.out if restart

[njzjz]

Summary

If LAMMPS restarts, model_devi.out should be appended to instead of creating a new file.

Detailed Description

How to implement:

1. set restartinfo to 1 (default)
   See: https://github.com/lammps/lammps/blob/9da49d9c6f521eed62ca0752297c9057d34d39db/src/pair.cpp#L61
2. add read_restart to pair, where open the file using std::ofstream::app mode
   See:
   https://github.com/lammps/lammps/blob/9da49d9c6f521eed62ca0752297c9057d34d39db/src/pair.cpp#L725-L729
   http://www.cplusplus.com/reference/fstream/ofstream/open/
3. add a condition to skip opening a new file and adding headers if restart
   See:

   deepmd-kit/source/lmp/pair_deepmd.cpp

   Lines 838 to 850 in f70437f

   	if (numb_models > 1 && out_freq > 0){
   	fp.open (out_file);
   	fp << scientific;
   	fp << "#"
   	<< setw(12-1) << "step"
   	<< setw(18+1) << "max_devi_v"
   	<< setw(18+1) << "min_devi_v"
   	<< setw(18+1) << "avg_devi_v"
   	<< setw(18+1) << "max_devi_f"
   	<< setw(18+1) << "min_devi_f"
   	<< setw(18+1) << "avg_devi_f"
   	<< endl;
   	}

Potential problems: Since we don't save restart files every timestep, there may be some duplicated data before and after restart.