Homogenization of code, new API for creating a Python object from shared ptrs.

doc/c_cpp_api.rst

xbob/learn/activation/activation.cpp

xbob/learn/activation/identity.cpp

xbob/learn/activation/include/xbob.learn.activation/api.h

xbob/learn/activation/linear.cpp

xbob/learn/activation/logistic.cpp

xbob/learn/activation/main.cpp

xbob/learn/activation/mult_tanh.cpp

xbob/learn/activation/tanh.cpp