Aging of network & beginning of GraphicNN

- modified methods in both Neuron and NeuronNetwork class in ordre to be
able to suppress Neurons in the Network to simulate aging. In that case,
the value of the pointer on a dead Neuron will be 0, so we have to check
everywhere wether or not the pointer to Neurons are 0.
- slightly modified the testLearningProcess to have the errorMean
- beginning of the graphic representation of a NeuralNetwork through the
GraphicNN class. Each Neuron is represented by a GraphicNeuron object.