add ML labeling option for vasp ml-aimd OUTCAR

[PKUfjh]

I fixed bugs in the previous PR #278.

This PR intends to add labeling option for VASP machine learning force field AIMD, the default setting is the same with the original repo, which selects the first principle calculation among the MD frames as energy, force and viral labels. But one can also add option in the LabeledSystem like

LabeledSystem(filename,fmt='vasp/outcar',ml=True)

to use the machine learned energy, force and viral labels.
This option obviously is not as accurate as the default fp labels, but the motivation for adding this option (for me) is mainly to sample the structures rather than use the labeled system as training set. And you only need to change a little bit to realize the function. Also, since the default setting is the same as the original one, there is little worry to use this option in a wrong way.

[codecov-commenter]

Codecov Report

Merging #281 (5eb4419) into devel (7d72651) will decrease coverage by 0.76%.
The diff coverage is 32.30%.

@@            Coverage Diff             @@
##            devel     #281      +/-   ##
==========================================
- Coverage   81.57%   80.81%   -0.77%     
==========================================
  Files          62       62              
  Lines        5173     5232      +59     
==========================================
+ Hits         4220     4228       +8     
- Misses        953     1004      +51     

Impacted Files	Coverage Δ
dpdata/vasp/outcar.py	67.27% <28.33%> (-29.13%)	⬇️
dpdata/plugins/vasp.py	98.48% <80.00%> (-1.52%)	⬇️

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