Use values instead of DataFrame for all modes of SequentialFeatureSelector

[daniel-sch]

Description

SequentialFeatureSelector now supports DataFrame as input for all operating modes (forward/backward/floating).

Related issues or pull requests

Fixes #505

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 @daniel-sch! Thanks for updating the PR.

• In the file mlxtend/feature_selection/sequential_feature_selector.py, following are the PEP8 issues :

Line 270:17: E117 over-indented

Comment last updated on February 13, 2019 at 14:37 Hours UTC

[coveralls]

Coverage increased (+0.03%) to 91.617% when pulling fdc0e5d on daniel-sch:fix_dataframe_backward_feature_selection into 1ca3059 on rasbt:master.

[rasbt]

Nice, thanks. This could also help with #502 then!