Add support for 'deepmd/mixed' format with dpdata.MultiSystems

[iProzd]

Add support for 'deepmd/mixed' format with dpdata.MultiSystems

1. Support dump from dpdata.MultiSystems to mixed type format:
   dpdata.MultiSystems.to_deepmd_mixed('dir_name_mixed')
2. Support load from mixed type format to dpdata.MultiSystems:
   dpdata.MultiSystems.load_systems_from_file('dir_name_mixed', fmt='deepmd/mixed')

[iProzd]

I've added unittest for this, but I'm wondering where to add the docs.

[codecov-commenter]

Codecov Report

Base: 82.39% // Head: 82.35% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (a0951a0) compared to base (18508af).
Patch coverage: 81.57% of modified lines in pull request are covered.

❗ Current head a0951a0 differs from pull request most recent head 17edd35. Consider uploading reports for the commit 17edd35 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #422      +/-   ##
==========================================
- Coverage   82.39%   82.35%   -0.04%     
==========================================
  Files          67       68       +1     
  Lines        5963     6182     +219     
==========================================
+ Hits         4913     5091     +178     
- Misses       1050     1091      +41     

Impacted Files	Coverage Δ
dpdata/format.py	88.33% <50.00%> (-1.33%)	⬇️
dpdata/deepmd/mixed.py	80.11% <80.11%> (ø)
dpdata/system.py	85.40% <83.78%> (-0.14%)	⬇️
dpdata/plugins/deepmd.py	67.21% <94.44%> (+4.71%)	⬆️

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[iProzd]

@wanghan-iapcm @njzjz

[njzjz]

The docs can be automatically generated in https://deepmodeling--422.org.readthedocs.build/projects/dpdata/en/422/formats.html

[wanghan-iapcm]

I find the UT is trivial: All systems has the same types and numbers of atoms. Please provide tests for the cases

• systems has the same total number of atoms but different atom types,
• systems has different total numbers of atoms,

and see if your code is working as it was designed.

The doc for the mix type is also missing. Please provide the doc.