Stopping condition of the feature space attack expects the model to give a score between -1 and 1

[danielgibert]

The early stopping condition in line 186 of feature_space_attack.py stops when the model returns a score lower than 0. For the SVM classifier there is no issue as it returns values between -1 and 1. However, for models that return scores between 0 and 1 it could affect the generation of adversarial samples.