Add --site-mixture/--split-site-mixture options to velph init

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[codecov]

Codecov Report

❌ Patch coverage is 95.74468% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.24%. Comparing base (2ff47c1) to head (e5c575d).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/phelel/velph/cli/init/init.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #182      +/-   ##
===========================================
+ Coverage    68.05%   68.24%   +0.19%     
===========================================
  Files           53       53              
  Lines         6370     6399      +29     
===========================================
+ Hits          4335     4367      +32     
+ Misses        2035     2032       -3     

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