MNIST and CIFAR10 image datasets added + implementation of convolutional neural networks added

Lupo Pasini, Massimilianorequested to merge
cnn_class_implementation into master

I created an abstract class called NeuralNetwork. Both MLPRegression and CNClassification are child classes that inherit from NeuralNetwork. In also introduced MNIST and CIFAR10 as benchmark dataset for convolutional neural networks.

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