Fixing of association rule lastest version

[zazass8]

Code of Conduct

Description

Related issues or pull requests

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• Ran PYTHONPATH='.' pytest ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend

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[zazass8]

Hello @rasbt, so this PR links to the issues that were foreseen in #1106. In this commit, I tried to fix num_itemsets that was initially required. Now, it is an optional argument and only to be used for the case when missing values exist in the dataset.

Furthermore, it was mentioned that there was excessive memory usage when the latest version of association rules was executed. For this, the following commit uses a simplified approach of nansum as mentioned in the previous PR. Now, it seems there's no issue when the unit tests are executed. I'd assume that also this concern, is fixed.

Let me know for any feedback.

[rasbt]

Thanks a lot, that's awesome. I will merge and make a new release!