add histogram which shows keys that are created (ensure that all gestures/reps/subjects participate in the training process)

[tkarab]

• [x] dict with all the available gesture keys, whose value is the number of times the gesture has been used in training
• [x] shuld be a member of TaskGenerator
• [x] Should be initialized after get__data()
• [x] each key should have a list of 2 elements, one for augmented and one for non augmented version of data used for each key
• [x] function in plot functions for plotting stacked bar with different colours for augmented and non augmented data
• [x] function in taskGenerator that calls the respective function of plot_functions

[tkarab]

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