Add locations of all PV systems, including those with no data

[jacobbieker]

Detailed Description

Right now, we save the data and location of PV systems that we have readings from. Adding the physical locations of all PV systems, including those we don't have live data from, could help the model better learn about which parts of certain gsps to focus on, etc.

Context

Possible Implementation

[JackKelly]

Good idea!

I think PVOutput.org has metadata for something like 2,000 PV systems in GB (so still a tiny subset of the ~1M PV systems installed in the country).

Related (but separate) issue: #184