Pass cosmetic attrs through to Potential objects

[SimonBoothroyd]

Description

Currently cosmetic attributes on force field parameters do not propagate through to a Potential objects parameters. Currently we need to re-look them up from the original force field.

It would be great if these could be passed through.

[SimonBoothroyd]

My current hacky workaround:

def to_interchange(ff, top):
    inter = openff.interchange.Interchange.from_smirnoff(ff, top)

    for col_k, col_v in inter.collections.items():
        for pot_k, pot_v in col_v.potentials.items():
            if " " not in pot_k.id:
                handler = ff[pot_k.associated_handler]
                param = handler[pot_k.id]
            else:
                smirks, lp, match = pot_k.id.split(" ")
                handler = ff["VirtualSites"]
                (param,) = handler.get_parameter(
                    {"smirks": smirks, "name": lp, "match": match}
                )

            if not hasattr(param, "_cosmetic_attribs"):
                continue

            for attr in param._cosmetic_attribs:
                pot_v.parameters[attr] = getattr(param, f"_{attr}")

    return inter

[mattwthompson]

Seems fair, I don't use cosmetic attributes so this never occurred to me.

I figure the last loop in that block of code should do the trick, and having it turned on by default makes sense

[mattwthompson]

@SimonBoothroyd Looking at this more closely now - does it make sense to have it on the Potential or PotentialKey object? Right now Potential.parameters really only stores the stuff that describes the physics of the interaction, whereas i.e. stuff like parameterize_eval shoved in for ForceBalance's accounting seem like things that better live on pointers to those objects. I'm not sure if such a physics vs. bookkeeping distinction holds for other use cases.