scikit-learn: Enhance cross validation and related modules.

by Raghav R V for Python Software Foundation

1. Make CV iterators data independent and provide a clean API with deprecations. 2. Group together/clean up/organize the model_selection module. 3. Multiple metric support to grid_search et al. 4. Extend sample_weight support to grid_search et al. 5. Generalized CV and early stopping 6. Introducing additional CV strategies for non trivial cv tasks 7. Make an extensive tutorial on CV. 8. Improve contributor documentation / Better docstrings.