GEOM* dREL? add it back?

[rowlesmr]

I was exploring why we used _model.id as a key for GEOM*, rather than _structure.id, and came across the OG PR discussion.

There was a comment in there about "adding the dREL back later"; should that be done? (also just to flag it in an open issue)

[jamesrhester]

Yes, eventually, let's keep this issue here, got a few more pressing things at the moment. That dREL is interesting, because it is essentially trying to create a connected molecular model, which might not be the one the authors decide on, so it is really an _enumeration.default, I think.