assign type map for LAMMPS pair_style deepmd

[njzjz]

Fix #2237. Use the LAMMPS pair_coeff command to set type_map, like ReaxFF, EAM, etc.
Add and improve the documentation about type_map. From now on, setting type_map explicitly is encouraged (but not necessary).

[codecov-commenter]

Codecov Report

Base: 75.65% // Head: 75.64% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (20851eb) compared to base (60955e2).
Patch coverage: 63.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2267      +/-   ##
==========================================
- Coverage   75.65%   75.64%   -0.02%     
==========================================
  Files         204      204              
  Lines       20267    20294      +27     
  Branches     1438     1446       +8     
==========================================
+ Hits        15334    15351      +17     
- Misses       4028     4030       +2     
- Partials      905      913       +8     

Impacted Files	Coverage Δ
source/lmp/pair_deepmd.cpp	52.24% <62.06%> (+0.43%)	⬆️
deepmd/utils/argcheck.py	90.36% <100.00%> (ø)
source/lib/include/ComputeDescriptor.h	76.87% <0.00%> (-0.23%)	⬇️
source/lib/src/fmt_nlist.cc	79.51% <0.00%> (+1.20%)	⬆️

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