Cross resonance Rabi experiment and rough amplitude calibration

[nkanazawa1989]

Summary

This PR adds characterization and calibration experiment for cross resonance pulse amplitude. It drives the control qubit at a frequency of the target qubit, and measure the population of the target qubit and fit the data with the oscillation analysis.

Details and comments

New experiment is written in the style recently I started to use in my project, aiming at future parametric circuit execution. If this looks good, I'll refactor other experiments in the followup.

Added features are

    parameter = Parameter("some_name")
    # class variable of the parameter object to use in this class

    def parameterized_circuits(self) -> Tuple[QuantumCircuit, ...]:
        # Return parameterized circuits

    def parameters(self) -> np.ndarray:
        # Return parameter values

    def _setup(self):
        # Update minor settings, such as axis name of plotter. 
        # This avoids overhead of overriding the constructor method just for updating options.
        # i.e. subclass may want different name for Y axis, or name of the returned quantity. 

Then the conventional BaseExperiment.circuits method consumes new parameterized_circuits and parameters output to build circuit. For example, the parameters method can consider the hardware constrains such as granularity. There is no API break with this implementation.

PR checklist (delete when all criteria are met)

• I have read the contributing guide CONTRIBUTING.md.
• I have added the tests to cover my changes.
• I have updated the documentation accordingly.
• I have added a release note file using reno if this change needs to be documented in the release notes.

[nkanazawa1989]

On hold until #971 is complete. I need a two qubit backend simulator to write end-to-end test.

[nkanazawa1989]

Bug fix for count dict key in 432219f. This commit also adds new fitter for cross resonance Rabi, since its oscillation is nonlinear because of the nonlinearity of the ZX term against the CR tone amplitude.

This notebook shows how this experiment can be used to bringup two qubit gate.
https://gist.github.com/nkanazawa1989/74d9d21f9f0757d6d44f0431b1cffb26

[CLAassistant]

All committers have signed the CLA.