Derive offline simulators circuits from API payload.

[airwoodix]

Summary

Use the API payload to derive the circuit submitted to the Aer simulator backend for offline simulators.

This ensures that the memory map is the same for the simulator and the remote devices and prevents double-application of the memory map in the offline simulators (by the simulator backend and the result logic in this package).

Resolves #85.

Details and comments

• simplify the qiskit -> aqt conversion by using the flattened quantum registers view provided by QuantumCircuit.find_bit. This is done in preparation of tackling Remove usage of deprecated Bit.index and Bit.register #80.
• the implementation of the aqt -> qiskit conversion is less elegant than that used in cloud portal due to the cumbersome generated models and the absence of pattern matching in Python 3.8-3.9.
• the OfflineSimulatorResource still uses the implicit knowledge that all circuits in the API payload have the same number of repetitions. This makes the simulator call simpler, but will lilkely change in the future.