Implement JSON serialization for estimators #32

[mariam851]

Hi @rasbt,

I hope you're doing well.

I have implemented the JSON serialization utilities for mlxtend as discussed in issue #32. My goal was to provide a reliable, human-readable alternative to pickle that avoids versioning conflicts and platform dependencies.

Why this implementation is robust:

Dynamic Reconstruction: Instead of requiring the user to manually instantiate a model before loading, I used importlib to store and recall the module and class. This makes the load_model_from_json function truly "smart" and polymorphic.

Custom Type Handling: I implemented a specialized MlxtendEncoder to bridge the gap between NumPy and JSON. It handles ndarrays, numpy scalars, and provides a safe fallback mechanism to prevent serialization crashes.

State Integrity: The implementation ensures that "fitted" attributes (identified by the trailing underscore _) are correctly cast back to NumPy arrays upon loading, preserving the exact state of the estimator for immediate inference.

Decoupled Design: By placing these utilities in mlxtend.utils.serialization, I ensured the logic is centralized and easily maintainable without bloating the individual estimator classes.

Validation:

Unit Tests: Added tests in mlxtend/utils/tests/test_serialization.py. Verified with Perceptron that the model's weights and predict output remain identical after a round-trip save/load.

Code Quality: The code has been linted and formatted using black, isort, and flake8 to match the project's standards.

I've put a lot of thought into making this extensible for other estimators in the library. Looking forward to your feedback!

Fixes #32

[rasbt]

Thanks for the PR. Could you also add a brief documentation for this? E.g., you could consider this other utils one as an example: https://github.com/rasbt/mlxtend/blob/master/docs/sources/user_guide/utils/Counter.ipynb to create a docs/sources/user_guide/utils/serialization.ipynb file (these jupyter notebooks are automatically converted to the web documentation each version release)

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[mariam851]

Hi @rabst,
I've updated the PR with the documentation notebook and refined the serialization utilities. The example in the user guide is now working perfectly. Ready for review!