Add Joint Model about satflow HOT 2 OPEN

Sounds good!

Let's use this new issue to discuss the joint model. I'll close some older issues where we've discussed related ideas:

• openclimatefix/predict_pv_yield#68
• openclimatefix/perceiver-pytorch#1 (comment)

As I mumbled about on the meeting this afternoon, I think one of the first steps towards the 'joint model' is to get the position encoding right for the recent history of the PV power data. For each measurement of historical PV power, we definitely want to encode the position in time and space, probably using the same encoding as for the satellite data (so the model can associate historical PV data with historical clouds: it can see "ooh, look, when that cloud passed over that PV system half an hour ago, it reduced power output by 50%").

We may also want the PV "position" encoding to encode an embedding of the ID of the PV system ("ooh, I remember that PV system was unusually dirty, so I'll compensate for that")

(Maybe that "PV-position-encoding" code should go into nowcasting_utils, so predict_pv_yield can use it?)

from satflow.

Yeah, I think breaking out the PV position encoding, and maybe the position encoding in general should probably go into nowcasting_utils I've opened one here https://github.com/openclimatefix/nowcasting_utils/issues/30

from satflow.