Save datasets to files

[jl-wynen]

Should be enough to serialize the models to JSON.

[nitrosx]

Could you please elaborate more on this functionality?

[jl-wynen]

Early on, we discussed that it can be useful to download datasets ahead of time and use them later on, e.g. on a plane w/o internet connection.

So my suggestion would be to add methods to Dataset to save to / load from JSON. This would mostly require serializing the models. Except File, where we need to be more careful because of File.local_path.

[nitrosx]

Now I remember. Let me know if we need to brainstorm again