Modified neural net activation to avoid the saturation point of the
activation function. The new weight values are |w_ij| < 1/sqrt(||F||).
	modified:   ml/neural_net.cpp
	modified:   ml/neural_net_activation.cpp
Added Object, and with it, clone() and str().
	modified:   ltr_common.hpp
	modified:   ml/linear_regression.cpp
	modified:   ml/linear_regression.h
	modified:   ml/ml_model.h
	modified:   ml/neural_net.cpp
	modified:   ml/neural_net.h
	new file:   object.h