Add parabolic errors to minuit output

[pelssers]

Since we run migrad in bestfit_minuit we already have the parabolic errors estimated. These errors are added to the output dictionary.

For more advanced error estimation we could consider running minos (maybe via minimizer_kwargs) which computes asymmetric errors using the likelihood.

[JelleAalbers]

Good idea! This dictionary is never used inside blueice, so we can add anything we want.