Introduce operator_to_vector and vector_to_operator

[ytdHuang]

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Description

The functions mat2vec and vec2mat are called operator_to_vector and vector_to_operator, respectively.

Therefore, I made them as synonyms.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.20%. Comparing base (300fd5d) to head (9a19798).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #413   +/-   ##
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  Coverage   94.20%   94.20%           
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  Files          46       46           
  Lines        2918     2918           
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  Hits         2749     2749           
  Misses        169      169           

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