Clean-df

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* Improve the performance when calling ``report`` method.
* The ``pytest`` now is including the full methods in the module.

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* Improve memory consumption and module performance.

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* Fix a bug that made "dict_keys" error in some speical cases.

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* Improve module performance.

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* Add a new report for categorical columns.
* Make the module more efficient.

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* Rectify and organize documentation.
* Provide test to GitHub Actions.