Enh/active learning loads from database
This merge request includes an entirely new implementation of the ActiveLearningBatchGenerator class.
Unlike the old implementation, which required all data to be handled in memory, the new ActiveLearningBatchGenerator class can read from an hdf5 database (though it is still possible to pass data via memory).
Also, the new ActiveLearningBatchGenerator class outputs an instance of BatchGenerator.
The train_active method in BasicCNN has been updated to accommodate the changes to the ActiveLearningBatchGenerator.
Should be ready to merge!