Add regression; Add bob tf trim; Other small modifications

bob/learn/tensorflow/dataset/bio.py

bob/learn/tensorflow/estimators/Logits.py

bob/learn/tensorflow/estimators/Regressor.py

bob/learn/tensorflow/estimators/__init__.py

bob/learn/tensorflow/network/JointIncResV2Simple.py

bob/learn/tensorflow/network/SimpleCNN.py

bob/learn/tensorflow/script/eval.py

bob/learn/tensorflow/script/predict_bio.py

bob/learn/tensorflow/script/trim.py

bob/learn/tensorflow/test/test_regression.py

conda/meta.yaml

doc/index.rst

doc/py_api.rst

doc/regression.rst

doc/user_guide.rst

setup.py