Save model info

[ElliottKasoar]

Resolves #327

For model_path (or equivalent inputs e,g, model), this adds mlip_model as an info label when structures are output.

For strings and paths, this is fine. For already-loaded torch models, it's less clear what the best label is, so I'm open to alternatives.

This will also interact with any implementation of #240, but we can deal with that when we come to it.

@alinelena, was the suggested info['{arch}_{model}'] = model replacing info["arch"] or additional? To me it doesn't make sense to have "{arch}_model" as well as "arch", which is how I originally interpreted it, but perhaps it makes sense to combine them into a single info label, which I just realised may be what you meant.