Make `Interchange.positions` the sole source of truth for positions/coordinates

[mattwthompson]

i.e. make Interchange.topology.{get|set|clear}_positions do nothing

Relates to #1282

I would like to know what @Yoshanuikabundi thinks about this idea

P.S. maybe a Interchange.coordinates alias would be useful?

[Yoshanuikabundi]

I think single sources of truth are good! I'm supportive of this change, though I'd prefer my proposal in #1282 as implementation. I think positions is the superior name as it's more specific - box vectors columns and velocities are both technically co-ordinates. I'm not convinced of the value of aliases, but they're not super costly.