Fix velph-selfenergy-generate (issue #28)

[atztogo]

This commit fixes issue #28.

Note that the command name velph-elph has been changed to velph-selfenergy. Previously, self-energy and transport calculations were variants of velph-elph, but they are now considered independent calculation types.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 19.56522% with 37 lines in your changes missing coverage. Please review.

Project coverage is 73.27%. Comparing base (79725c8) to head (2c84432).
Report is 25 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/phelel/velph/cli/selfenergy/generate.py	5.55%	17 Missing ⚠️
src/phelel/velph/cli/hints/__init__.py	0.00%	16 Missing ⚠️
src/phelel/velph/cli/selfenergy/__init__.py	60.00%	2 Missing ⚠️
src/phelel/velph/cli/transport/__init__.py	50.00%	1 Missing ⚠️
src/phelel/velph/cli/transport/generate.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #29      +/-   ##
===========================================
- Coverage    73.42%   73.27%   -0.16%     
===========================================
  Files           74       75       +1     
  Lines         5223     5320      +97     
===========================================
+ Hits          3835     3898      +63     
- Misses        1388     1422      +34     

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