remove the dependency of property num_atoms on the cube file content

[robinzyb]

remove the dependency of property num_atoms on the cube file content. When writing a new cube file, the num_atoms property is obtained from self.stc which is ase Atoms object.

[codecov]

Codecov Report

Attention: Patch coverage is 73.52941% with 9 lines in your changes missing coverage. Please review.

Project coverage is 43.60%. Comparing base (221ecdc) to head (5e512e3).
Report is 20 commits behind head on main.

Files	Patch %	Lines
cp2kdata/cube/cube.py	73.52%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   43.56%   43.60%   +0.04%     
==========================================
  Files          32       32              
  Lines        1988     2002      +14     
==========================================
+ Hits          866      873       +7     
- Misses       1122     1129       +7     

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