Implement the feature importance for Decision Tree Classifier

[tushushu]

This PR fixes part 1/3 of #124

Checklist

• My branch is up-to-date with development branch.
• Everything works and tested on latest stable Rust.
• Coverage and Linting have been applied

Current behaviour

• The impurity is not available in the Node object
• The number of samples is not available in the Node object
• The number of features is not available in the Decision Tree Classifier
  So the feature importance cannot be calculated after model is trained.

New expected behaviour

The feature importance can be calculated after model is trained.

Change logs

The feature importance can be calculated after the DecisionTreeClassifier is trained.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (886b563) 44.98% compared to head (42c4949) 45.17%.

Files	Patch %	Lines
src/tree/decision_tree_classifier.rs	60.71%	11 Missing ⚠️

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Additional details and impacted files

@@               Coverage Diff               @@
##           development     #275      +/-   ##
===============================================
+ Coverage        44.98%   45.17%   +0.18%     
===============================================
  Files               85       85              
  Lines             7214     7226      +12     
===============================================
+ Hits              3245     3264      +19     
+ Misses            3969     3962       -7     

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[Mec-iS]

hi! thanks for the PR.

Please follow the guidance about running the linter in CONTRIBUTING. The linter check is failing.

[tushushu]

hi! thanks for the PR.

Please follow the guidance about running the linter in CONTRIBUTING. The linter check is failing.

Hey, thanks for providing the doc. I was aware of that and changed the PR status as "Draft". Let me try to fix the linter today or tomorrow.

[tushushu]

Hey @Mec-iS I have fixed the linter error, and would you like to review the PR?

[tushushu]

Is there anyone else can help review the PR? It has been quite a while.

[Mec-iS]

Thanks @tushushu ! Sorry I missed the notification for this.

[tushushu]

@Mec-iS Nice working on this PR.