Disable input checking in StackingCVClassifier to support pipelines

[rasbt]

Description

Disables input validation for StackingCVClassifier because it causes issues if pipelines are used as input.

Related issues or pull requests

Fixes #605

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

[coveralls]

Coverage decreased (-0.002%) to 92.347% when pulling 297fbe6 on sclf-warn into 98fe01f on master.