Unify names of X_binned, features_data and binned_features

[NicolasHug]

X_binned in GradientBoosting is named features_data in the grower and binned_features in the splitting context.

How about we use instead:

• X_binned in every class
• or X_binned in GradientBoosting and binned_features in both the grower and splitting context

?

As a side note, passing n_features to the splitting context is redundant since it's just binned_features.shape[0].