Defining the new Backend API classes

[HGSilveri]

• Introduces the Results class and make Result a subclass of it in backwards compatible way
• Defines the State and Operator ABCs
• Updates BackendConfig and defines EmulationConfig
• Defines the EmulatorBackend ABC
• Defines the Callback and Observable ABCs
• Defines the default observables

To do:

• Define QutipState to have an example
• Idem for QutipOperator
• Solve relevant TODOs
• Write UTs

[a-quelle]

why is this a np.array but below you use AbstractArray?

[HGSilveri]

You mean for interaction_matrix? I use AbstractArray because I expect people might want to differentiate against it

[a-quelle]

why do we not simply throw an error if the eigenbasis is not predefined? No backend would know how to simulate it anyway.

[HGSilveri]

I'm not sure I get what you mean. Do you mean in the case that the eigenstates don't match one of these sets we would just throw an error right away?