Fix bug when reading file in "pwmat/movement" format

[lhycms]

PWmat adds nonperiodic_Position block to MOVEMENT file, used to output the atomic positions without non periodic boundary conditions.
Due to the fact that dpdata previously locates the number of atoms based on the keyword Position, when MOVEMENT contains nonperiodic_Position, it will cause the number of atoms to double, resulting in a bug.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (18a0ed5) 82.81% compared to head (70f5cd5) 83.19%.
Report is 12 commits behind head on devel.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #599      +/-   ##
==========================================
+ Coverage   82.81%   83.19%   +0.38%     
==========================================
  Files          73       76       +3     
  Lines        6580     6789     +209     
==========================================
+ Hits         5449     5648     +199     
- Misses       1131     1141      +10     

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