Implement Loadable interface for exported fit results

[redeboer]

I think it would be good if we would define an additional interface/protocol (maybe Loadable) that specifies the loading of the parameters. Then Summary classes that also have a ability to load the parameters from the summary file would inherit from that interface. That would also make the UI simpler.

The interface would contain something like this

def load_latest_parameters(filename: str) -> dict:

Originally posted by @spflueger in #159 (comment)

[redeboer]

Closed by #177