Expose fit parameters in `FitTrace`?

(Follow-up of #128.) `FitTrace` currently exposes the parameters of the fitted trace through its `trace_model_fit` attribute. Should it also expose other information about the fit, like uncertainties?

Review follow-up issue #128 and inspect FitTrace's existing trace_model_fit attribute to determine what fit information is currently available. Define which additional information, such as uncertainties, should be exposed and what the completed public interface should guarantee before implementation.

Written by the indexing model from the issue text.