Implementing methods to train the network via GUI.

Also changed the name of data class to gestion_data ( conflict with Qt
widget )
(Not tested because images_data might be wrong)
-in Network:
data_training(void) will train the network with the data we have,
provided the network is of correct dimensions ( 784 on first layer, 10
on last layer )
- in extract_data:
output_data to convert labels into wanted values for the output of the
network