Introduce convert_unit

[ytdHuang]

Checklist

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Description

Summarize of this PR:

• Introduce convert_unit, which is the similar function in qutip (see here for the github code)
• add runtests (and also type inferrence tests) for utility functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.51%. Comparing base (bde9142) to head (eeb796f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
- Coverage   94.48%   93.51%   -0.98%     
==========================================
  Files          31       31              
  Lines        2267     2297      +30     
==========================================
+ Hits         2142     2148       +6     
- Misses        125      149      +24     

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