support mapping to ghost type

[link89]

Currently pair_coeff support customizing mapping rule by the following way

pair_coeff * * H O H

But there are cases that need to mapping specific atom type to ghost type, or type that doesn't existed in deepmd.

This PR addresses the issue by treating NULL as ghost type, so that users can specify atoms type to be ghost type by the following way

pair coeff * * H O NULL

[njzjz]

Other pair styles in LAMMPS use NULL as the placeholder, for example, https://docs.lammps.org/pair_eam.html and https://docs.lammps.org/pair_reaxff.html. It's preferred to use the same key.
Documentation should be added like two above pair styles.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +2.16% 🎉

Comparison is base (9a2cc91) 78.72% compared to head (dbcfe4f) 80.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2732      +/-   ##
==========================================
+ Coverage   78.72%   80.89%   +2.16%     
==========================================
  Files         241      209      -32     
  Lines       25582    20749    -4833     
  Branches     1517        0    -1517     
==========================================
- Hits        20140    16784    -3356     
+ Misses       5050     3965    -1085     
+ Partials      392        0     -392     

see 73 files with indirect coverage changes

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[link89]

Update the patch will new placeholder NULL and its usage.