Fix MGP issues & Add LAMMPS pair styles

[YuuuXie]

• close no "mgp" or "mgpf" pair_style in lammps #164: no "mgp" or "mgpf" pair_style in lammps
  • now added to folder mgp/pair_styles folder
• close MGP does not handle 0 cutoff #167: MGP does not handle 0 cutoff
• close OTF parser fails at "Cutoffs: {'twobody': 3.0, 'threebody': 5.0}" #205: MGP Fails to run for isolated atoms
• close mgp.py: raise ValueError(rebuild_dict, newbound_dict, err_str) #208: mgp.py: raise ValueError(rebuild_dict, newbound_dict, err_str)
• Add mgp tutorial to docs, lmp code tutorials