Update plugins to work with interchange

[jthorton]

In order to work with interchange and the latest openff toolkits we need to update our plugins to follow the new interchange format. Local coordinate site virtual sites might be the most difficult to support.

[mattwthompson]

In recent versions, I'm moving it away from using LocalCoordinatesSite except for complicated cases that are hard to work out by hand.

I need to document this when the transition is complete, but off the top of my head

• BondCharge -> TwoParticleAverageSite
• Four-site water -> ThreeParticleAverageSite
• Five-site water -> OutOfPlaneSite
• TrivalentLonePair -> OutOfPlaneSite
• Planar context (aromatic nitrogens, some carbonyls, etc.) -> ThreeParticleAverageSite
• Non-planar (and non-water) might use LocalCoordinatesSite but are meant to end up with OutOfPlaneSite

Depending on the chemistries being studied, this might be possible now or soon.