Updated triplet trainer

bob/learn/tensorflow/datashuffler/Base.py

bob/learn/tensorflow/datashuffler/Memory.py

bob/learn/tensorflow/datashuffler/OnlineSampling.py

bob/learn/tensorflow/datashuffler/Siamese.py

bob/learn/tensorflow/datashuffler/SiameseDisk.py

bob/learn/tensorflow/datashuffler/SiameseMemory.py

bob/learn/tensorflow/datashuffler/TripletDisk.py

bob/learn/tensorflow/datashuffler/TripletMemory.py

...n/tensorflow/datashuffler/TripletWithFastSelectionDisk.py

...learn/tensorflow/datashuffler/TripletWithSelectionDisk.py

bob/learn/tensorflow/loss/TripletLoss.py

bob/learn/tensorflow/script/train_mobio.py

bob/learn/tensorflow/script/train_siamese_casia_webface.py

bob/learn/tensorflow/trainers/Trainer.py

bob/learn/tensorflow/trainers/TripletTrainer.py