PVDAQ sites for reference data

[wholmgren]

We need to add some of the PVDAQ sites to the reference data set. The data does not need to be available in real time - anytime since April 2019 (the start of the reference forecasts archive) would be good enough.

https://developer.nrel.gov/docs/solar/pvdaq-v3/

https://maps.nrel.gov/pvdaq/?aL=su70DP%255Bv%255D%3Dt&bL=clight&cE=0&lR=0&mC=40.212440718286466%2C-91.58203125&zL=4

There's a stale pvlib python PR for the PVDAQ api. Maybe start there.

pvlib/pvlib-python#664

[wholmgren]

I've sorted through the public PVDAQ database and narrowed it down to the following locations based on data availability and quality. PVDAQ is blue and RTC is purple.

It's a big expansion of the reference data set in terms of MW. The RTC sites sum to ~10 kW while the PVDAQ sites sum to 3.8 MW. Data ranges from 15 seconds to 15 minute intervals. It's not clear what the 15 minute intervals represent, though. There are also some issues with non-uniform time reporting to sort through.

It's a disappointing expansion in terms of climate zones - it doesn't actually add to the total covered (4). But it's still nice to add Golden and NJ. And the clustered sites might be useful for some people. Update: As mentioned in #436, there are a couple of SRML PV sites that we missed, so that gets us to 5 climate zones. Huzzah.

We might be able to use PVDAQ for the RTC sites instead of pulling them from Sandia's dashboard (Vermont doesn't show up on the map below because its missing data for the last year). I suggest addressing that with a separate issue once we're comfortable with PVDAQ.