Is your feature request related to a problem? Please describe.

It is not clear to me what value ModelChain has in comparison to the PVSystemLocalized class.

Describe the solution you'd like

Let's discuss if the codebase can be simplified by removing one or the other of these classes, which appear (at least on the surface) to overlap significantly in purpose, e.g., computing performance for a PVSystem at a given Location.

Describe alternatives you've considered

I'm leaning towards getting rid of ModelChain and making PVSystemLocalized the workhorse, but I really don't know, because I don't really know what workflow might be more popular among users.

Additional context

See the architecture discussion in #487.