Extend method for ptrace and entanglement

[ytdHuang]

This is just an extend method for lazy people like me.
If we are doing partial trace and leaving only one sub-system, we can specify sel::Int only (users don't need to add the square brackets [] to generate a Vector by themselves).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.40%. Comparing base (cd0bfa6) to head (5c7911c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   89.39%   89.40%   +0.01%     
==========================================
  Files          16       16              
  Lines        1565     1567       +2     
==========================================
+ Hits         1399     1401       +2     
  Misses        166      166              

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[albertomercurio]

Great!