fix bug of to_ase_structure in #98

[njzjz]

fix #98

[codecov-io]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.66%. Comparing base (5086a51) to head (7aea226).
Report is 529 commits behind head on devel.

Files with missing lines	Patch %	Lines
dpdata/system.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel      #99      +/-   ##
==========================================
+ Coverage   85.60%   85.66%   +0.05%     
==========================================
  Files          28       28              
  Lines        3175     3173       -2     
==========================================
  Hits         2718     2718              
+ Misses        457      455       -2     

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