Change keyword argument solver to alg

[ytdHuang]

Checklist

Thank you for contributing to HierarchicalEOM.jl! Please make sure you have finished the following tasks before opening the PR.

• Please read Contributing to Quantum Toolbox in Julia.
• Any code changes were done in a way that does not break public API.
• Appropriate tests were added and tested locally by running: make test.
• Any code changes should be julia formatted by running: make format.
• All documents (in docs/ folder) related to code changes were updated and able to build locally by running: make docs.

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Description

As title.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.70%. Comparing base (91e12c9) to head (d9ca561).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
+ Coverage   91.67%   91.70%   +0.03%     
==========================================
  Files          25       25              
  Lines        1538     1544       +6     
==========================================
+ Hits         1410     1416       +6     
  Misses        128      128              

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