Introduce sqrtm and fidelity

[ytdHuang]

Summary of this PR:

• improve LinearAlgebra.sqrt to support SparseArrays (by using sparse_to_dense before sqrt)
• introduce sqrtm (same as the one in qutip, only works for Operator)
• introduce fidelity

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.23%. Comparing base (6c0f46d) to head (fe1d450).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   91.12%   91.23%   +0.11%     
==========================================
  Files          21       21              
  Lines        1757     1769      +12     
==========================================
+ Hits         1601     1614      +13     
+ Misses        156      155       -1     

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