drop_last_proba feature for stacking classifiers

[rasbt]

Description

Adds a parameter to StackingClassifier and StackingCVClassifier to optionally drop one of the probability columns of the 1st-level classifiers to avoid collinearity.

Related issues or pull requests

Fixes #527

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 nosetests ./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., nosetests ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend

[pep8speaks]

Hello @rasbt! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-05-12 04:45:59 UTC

[coveralls]

Coverage increased (+0.04%) to 91.62% when pulling caa9c71 on drop-proba into 8a03d0c on master.