[Refactoring] [Testing] Noise

[chMoussa]

Folowing the digital noise MR #563, a few extra tasks are to be carried out:

• Properly making consistent the DigitalNoise with the Noise class and etc... Some are backend independent (Readout), others are analog/digital-specific. It makes sense to separate the different types.
• Including the noise in each block string: if added, check how it behaves with visualization.
• Serialization: make _to_dict and _from_dict functions to noise class
• Add tests! For transpiler functions such as set_noise

[chMoussa]

At the same time, tackle #369

[mlahariya]

@chMoussa closing this as most things are already implemented. Visualization which is left will be added to mega visualization issue. #613