Add Minuit instance to FitResults

[alexander-held]

Adding the iminuit.Minuit instance to the FitResults container would allow interacting with that directly to access more of the iminuit functionality. This could be useful if users want to run other steps after a fit and would work similarly to return_result_obj=True in pyhf.infer.mle.fit.

[MoAly98]

Hi @alexander-held, is this deeper than updating the class for FitResults and adding the instance returned from the fit? Just not sure what you intend by specifying that it would work similarly to the return_result_obj=True -- do you mean it should be optional?

I'm happy to be assigned this issue.

[alexander-held]

Great, thanks for helping with this! Thinking about it a bit more, I see no downside to just always adding it to FitResults so we don't need to make this optional. The difference with pyhf is that the fit function there returns a varying amount of objects depending on its keyword arguments but we always only return a single FitResults object so users who do not need that Minuit instance can just ignore it.