Enhanced cu function with double precision support

[TendonFFF]

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.
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Description

Add optional word_size argument, either Val(64) for double precision (default) or Val(32) for single precision.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.82%. Comparing base (623f88f) to head (9ab683c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/HierarchicalEOM_CUDAExt.jl	75.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   91.80%   91.82%   +0.02%     
==========================================
  Files          24       24              
  Lines        1513     1517       +4     
==========================================
+ Hits         1389     1393       +4     
  Misses        124      124              

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[ytdHuang]

LGTM !