Added ActorCrtic, AdvantageActorCritic and AsynchronousAdvantageActorCritic models.
Refactored neural network model code so that forwardPropagate() and backPropagate() can be used independently. Also make the codes easier to understand.
Made some fixes with the NeuralNetwork model.
Added new activation functions to NeuralNetwork model: Gaussian, SiLU, Mish and BinaryStep.