Enhancement added zhangs metric in association rules

[zuari1993]

Description

This PR adds the Enhancement requested in #980 . Fixes #980

Pull Request Checklist

• [x] Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• [x] Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• [x] Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• [x] 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)
• [x] Checked for style issues by running flake8 ./mlxtend

What I have done :

• Added Zhangs metric
• Added appropriate test functions to include the same

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Codecov Report

Base: 77.43% // Head: 77.46% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (78c3b57) compared to base (423d217). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #992 +/- ## ========================================== + Coverage 77.43% 77.46% +0.03% ========================================== Files 198 198 Lines 11165 11171 +6 Branches 1406 1406 ========================================== + Hits 8646 8654 +8 + Misses 2305 2304 -1 + Partials 214 213 -1 ``` | [Impacted Files](https://codecov.io/gh/rasbt/mlxtend/pull/992?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [.../frequent\_patterns/tests/test\_association\_rules.py](https://codecov.io/gh/rasbt/mlxtend/pull/992/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9mcmVxdWVudF9wYXR0ZXJucy90ZXN0cy90ZXN0X2Fzc29jaWF0aW9uX3J1bGVzLnB5) | `100.00% <ø> (ø)` | | | [mlxtend/frequent\_patterns/association\_rules.py](https://codecov.io/gh/rasbt/mlxtend/pull/992/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9mcmVxdWVudF9wYXR0ZXJucy9hc3NvY2lhdGlvbl9ydWxlcy5weQ==) | `100.00% <100.00%> (ø)` | | | [mlxtend/evaluate/counterfactual.py](https://codecov.io/gh/rasbt/mlxtend/pull/992/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9ldmFsdWF0ZS9jb3VudGVyZmFjdHVhbC5weQ==) | `100.00% <0.00%> (+6.89%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka)

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[rasbt]

Was just about to make these fixes to this PR. Thanks so much for taking care of it already!