Add flux-tunable transmon

[nulinspiratie]

The current Transmon class (quaqsim.architectures.transmon.Transmon) is currently a fixed-frequency transmon qubit, i.e. it does not have a dedicated flux-bias line. Adding a flux-bias line would enable the simulation of flux pulses, which could then be used for simulating flux-pulse two-qubit gates

Requirements

• The transmon with flux driving should be a FluxTunableTransmon
• The response profile can can be fixed to begin with as an asymmetric transmon, but should later support many types (see this paper, Fig. 2)
  

Acceptance Criteria

1. Qubit Spectroscopy vs. Flux: In practice, this experiment should map out the qubit frequency vs. flux profile specified by the capacitive-shunting modality (e.g. symmetric transmon, asymmetric transmon) as shown in Fig. 2 of this paper.

• Note 1: Typically this is run before the Rabi oscillation time/power is calibrated, and so a saturation pulse is used to create decoherence on resonance, which creates the on-resonant response of the qubit. Since this is a simulated environment, we know the perfect pi-pulse parameters, and can instead perform pi-pulse spectroscopy (replace the saturation operation with a pi-pulse).
• Note 2: set_dc_offset and update_frequency commands should be implemented to run this QUA experiment. Follow the example of other instructions such as FrameRotation2Pi in how they are compiled. Also, make use of the qiskit.pulse documentation for the final stage of simulation. For example, here is the ShiftFrequency instruction documentation.