Enable DataHandler to save nested dictionaries of numpy arrays

[arthurostrauss]

In some use cases, one is interested to save data types related to different qubits such as:

my_data = {"I": {"q1": np.array([...]),
                              "q2": np.array([...]),
                               ....}}
DataHandler.save_data(my_data)

At the moment, this will fail because numpy array is not directly serializable in JSON, and is not parsed correctly as it is done for the not nested case.

One should therefore be able to build according references in a nested fashion like in this example.