Bloch sphere implementation

[niccololaurora]

I added a visualization method which implements a Bloch Sphere. This tool could be useful to plot the final states of a classification task. Example:

state = np.array([1, 0], dtype="complex") b_sphere = Bloch() b_sphere.add_vector(state, mode="vector", color="black") b_sphere.save_plot("b_sphere.pdf") b_sphere.plot()