feat: access level

[ningkaiqiang]

Modify the access level of models and algorithms.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.47%. Comparing base (9e0bc91) to head (da1c9d0).

Files with missing lines	Patch %	Lines
Sources/FSRS/Models/FSRSModels.swift	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   89.43%   89.47%   +0.04%     
==========================================
  Files          11       11              
  Lines        1041     1045       +4     
==========================================
+ Hits          931      935       +4     
  Misses        110      110              

Flag	Coverage Δ
unittests	89.47% <85.71%> (+0.04%)	⬆️

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[ishiko732]

LGTM

[L-M-Sherlock]

LGTM