introduce rand_unitary and isunitary

[ytdHuang]

This PR introduces two functions which are the same ones in qutip:

• rand_unitary: Returns a random unitary with two different provided methods.
• isunitary: Test whether a Qobj $$U$$ is unitary operator. This function calls Base.isapprox to test whether $$U U^\dagger$$ is approximately equal to identity operator.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.26%. Comparing base (5499a17) to head (95e0977).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
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+ Coverage   93.21%   93.26%   +0.05%     
==========================================
  Files          28       28              
  Lines        2152     2168      +16     
==========================================
+ Hits         2006     2022      +16     
  Misses        146      146              

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