The neural net converted to use the Eigen library.
	modified:   ml/neural_net.cpp
	modified:   ml/neural_net.h
test.cpp added (temp!).
	modified:   Makefile
	new file:   test.cpp
Put the activation function into its own class. For some reason it
doesn't work; fix it!
	modified:   Makefile
	modified:   ml/neural_net.cpp
	modified:   ml/neural_net.h
	new file:   ml/neural_net_activation.cpp
	new file:   ml/neural_net_activation.h