Finish implementing T2starFitter parameter guesses

[aeddins-ibm]

What is the expected behavior?

qiskit.ignis.characterization.coherence.fitters.T2StarFitter has a nice looking method called guess_params to use fft to educate the initial guesses of the T2star fitter. However, there does not currently seem to be any reasonable way to call this method before performing the fit, since the fit is performed when the T2StarFitter is initialized, but AFAICT guess_params cannot be run before initializing the Fitter. I might just be missing something, but it looks like in order to use this function, you would have to create a dummy T2StarFitter object with your data and also some dummy parameter guesses/bounds, ask that dummy Fitter to run guess_params, and then create a second T2StarFitter object, feeding in those results as the parameter guesses.

Maybe one solution would be adding an auto_guess boolean kwarg for init? If it's True, then guess_params is called, and the p0 arg is overwritten with the results before the fit is performed. In this case the user should be allowed to leave p0 as None. Alternatively p0 could be set to auto.

Or if it makes more sense, could the function could be exposed directly as a utility somewhere, without belonging to any particular class?