add fixed feature set support to SFS

[rasbt]

Description

Adds a new fixed_features parameter to SFS for features to be forced to be included in the results.

Related issues or pull requests

#558

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 increased (+0.09%) to 92.35% when pulling cd5e660 on fixed-features into 936a96a on master.

[gosuto-inzasheru]

This parameter is not yet mentioned in the documentation at http://rasbt.github.io/mlxtend/user_guide/feature_selection/SequentialFeatureSelector/

[rasbt]

That's because it's currently only in the development branch (v 0.18.0dev) The latest release version os 0.17.0. The documentation will be updated automatically once 0.18.0dev is changed to 0.18.0 and uploaded to PyPI