Add check for existing equilibrated boxes

[lilyminium]

Is your feature request related to a problem? Please describe.

Not essential to the parent issue but extremely nice to have would be some kind of initial query for each EquilibrationLayer that looks inside the storage backend for existing equilibrated boxes, and short-circuits if they're found. That way users could re-run an EquilibrationLayer without worrying it would waste compute time.

Describe the solution you'd like

One way to do it could be by adding an initial check or protocol, perhaps, that checks for the output_to_store. Not sure if that's the best way.

Describe alternatives you've considered

Additional context