Export all info from a `FitResult` with a `Loadable`

A [`Loadable`](https://tensorwaves.readthedocs.io/en/0.3.7/api/tensorwaves.optimizer.callbacks.html#tensorwaves.optimizer.callbacks.Loadable.load_latest_parameters) callback currently only writes parameter values and errors, but not all info of a [`FitResult`](https://tensorwaves.readthedocs.io/en/0.3.7/api/tensorwaves.interface.html#tensorwaves.interface.FitResult).

Problem noted by @Leongrim 