Fix generating phonon-supercell files in velph

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Codecov Report

Attention: Patch coverage is 60.00000% with 22 lines in your changes missing coverage. Please review.

Project coverage is 73.51%. Comparing base (79725c8) to head (6a82b55).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/phelel/velph/cli/utils.py	0.00%	11 Missing ⚠️
src/phelel/velph/cli/supercell/generate.py	11.11%	8 Missing ⚠️
src/phelel/velph/cli/phono3py/init.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #18      +/-   ##
===========================================
+ Coverage    73.42%   73.51%   +0.08%     
===========================================
  Files           74       74              
  Lines         5223     5233      +10     
===========================================
+ Hits          3835     3847      +12     
+ Misses        1388     1386       -2     

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