feat(gaussian)!: Redesign API

[tasshi-me]

This PR redesigns gaussian module following Rust API Guideline [en, ja].

resolves TASSH-1

Checklist

• create Gaussian struct
• organiza functions under gaussian module with Gaussian struct
• depricate gaussian::{val, fit}
• documentation

Checklist (API Guideline)

• as_, to_, into_ (C-CONV)
• Getter (C-GETTER)
• implement standard convert traits (C-CONV-TRAITS)
• serde (C-SERDE)
• use ? in example (C-QUESTION-MARK)
• Link all the things (C-LINK)
• publish intermediates (C-INTERMEDIATE)
• generic float type arguments (C-GENERIC)
• impl Debug for all public types (C-DEBUG)
• re-export public dependencies (C-REEXPORT)

[codecov]

Codecov Report

Merging #15 (1f23247) into main (92d5ded) will decrease coverage by 6.04%.
The diff coverage is 89.26%.

❗ Current head 1f23247 differs from pull request most recent head cf65408. Consider uploading reports for the commit cf65408 to get more accurate results

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   97.95%   91.91%   -6.05%     
==========================================
  Files           5        6       +1     
  Lines         294      470     +176     
==========================================
+ Hits          288      432     +144     
- Misses          6       38      +32     

Impacted Files	Coverage Δ
src/gaussian/gaussian.rs	79.88% <79.88%> (ø)
src/gaussian/operations.rs	100.00% <100.00%> (ø)
src/lib.rs	100.00% <100.00%> (ø)
src/linalg.rs	100.00% <100.00%> (ø)

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