Fix ase.io.Trajectory compatibility

[frostedoyster]

Metatomic ASE calculators can't handle ase.io.Trajectory unless they are loading their model from a file.

@Luthaf I'm asking you for a "post-merge" review on this one... are you sure we can't handle this case directly in the metatomic calculator and somehow make it compatible with todict()?

[Luthaf]

are you sure we can't handle this case directly in the metatomic calculator and somehow make it compatible with todict()?

We could, but the two options both have issues:

• if we do store the model inside the dict, the per-step dict becomes enormous, and blows up the trajectory file
• if we do not store the mode inside the dict, this will lead to errors when trying to reload the trajectory and recompute properties (which is the intended use case).

We could maybe integrate a similar fix to what you do here, saving models to a temporary files, but this could create some weird errors if the temporary file is removed between saving and reloading.

Another solution would be to override .todict() for the flashmd calculator, and change the behavior here!