Add support for sparse matrices to StackingCVRegressor if use_features_in_secondary=True

[rasbt]

Description

add support for sparse matrices to StackingCVRegressor if use_features_in_secondary=True

Related issues or pull requests

Fixes #412

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

[coveralls]

Coverage increased (+0.05%) to 91.201% when pulling 8a971f8 on stackingcvregressor-sparse into 69b9fbe on master.