Adding CoTraining class implementation

[stompsjo]

This introduces a new class called CoTraining which can be used for semi-supervised co-training using logistic regression. This includes typical scikit-learn-esque methods like train and predict as well as methods for hyperparameter optimization and saving the model to file.

[coveralls]

Pull Request Test Coverage Report for Build 3949884919

• 92 of 92 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 95.673%

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Totals
Change from base Build 3932879255:	1.2%
Covered Lines:	398
Relevant Lines:	416

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💛 - Coveralls

[stompsjo]

@gonuke I updated this PR as a result of the changes made in PR #41 (hopefully I didn't forget anything). This is now ready for review.

[stompsjo]

Closing in favor of #49