System.out.println("out: " + n1_1_out );
// ------ test: Middle Layer > Neuron 2 ----------
input_layer_neuron_2.calcOutput();
middle_layer_neuron_2.calcOutput();
double n2_1_out = middle_layer_neuron_2.getOutput();
// System.out.println("> CalcOutput --------\n\n");