This commit changes all global function to static methods.
This commit changes all global function to static methods.

The global functions is_tomek, make_samples, in_danger and
is_noise, are now static methods. I believe it makes more sense
this way.

fmfn/UnbalancedDataset,dvro/imbalanced-learn,dvro/UnbalancedDataset,dvro/UnbalancedDataset,scikit-learn-contrib/imbalanced-learn,implus/UnbalancedDataset,scikit-learn-contrib/imbalanced-learn,yangspeaking/UnbalancedDataset,glemaitre/UnbalancedDataset,ningyuwhut/UnbalancedDataset,glemaitre/UnbalancedDataset,fmfn/UnbalancedDataset,dvro/imbalanced-learn