Describe the bug

Theoretically I should be able to re-fit virtual sites with Evaluator, but I can't because I can't retrieve a virtual site parameter by SMIRKS. This is by design as SMIRKS does not uniquely identify a VirtualSite parameter -- multiple are allowed to have the same SMIRKS. Therefore another solution is necessary. Interchange has VirtualSiteKeys.

To Reproduce

Apologies, I have no small MRE right now; I'm running into bugs while trialling a 4-site water model fit with ForceBalance. The problematic code is in system_subset:

When the parameter key being fit is a virtual site, this triggers the NotImplementedError linked in the openff toolkit.

Additional context

While pushing through getting a 4-site fit off the ground, I quickly patched this code as shown here: aa77c20

This is hacky as ParameterKey.smirks was probably not intended to hold all these attributes, but there were no other fields in ParameterKey to easily slot this on. It's also hardcoded to match corresponding patches I put in ForceBalance here and here.

This hardecoded vsite key was constructed to broadly approximate the Interchange version. The best solution would actually somehow incorporate a more canonical version of Interchange's VirtualSiteKey. However, I'm not certain the best way to, especially if ParameterKey.smirks needs to be a str.