Reflection about the fake backends

[yaelbh]

We have two types of fake backends:

• Those that are used in the spectroscopy and Rabi tests, and only look at the circuit's metadata.
• Those that are used in T1 and T2* tests, and look at the circuit's instructions, half-simulating them (still under assumptions on the circuit's structure).

Not sure which one to prefer. The first type is simpler, and second type half-tests the circuits.

[yaelbh]

T1 and T2* mock backends will hopefully not stay with us for long (#438).

Here is a general guideline for approaching mock backends:

1. Avoid mock backends when possible.
2. If a mock backend is required, I prefer an approach that's similar (but not identical) to what's currently done in T1 and T2*. Identify a subset of the Bloch sphere where the qubit always lies, namely, where the experiment circuits + the error can take the qubit. Then the mock backend is a simple simulator that supports only this part of the Bloch sphere, the set of gates that appear in the circuits, and the simulated error. For example, for the Hahn echo experiment (T2 experiment with Hahn echoes  #365) this would be (if I'm not mistaken) the XY and YZ circles.
3. Take the Rabi or Discriminator approach only if (1) and (2) are not possible.

Closing, because at least for me the picture is quite clear now.