Handle missing GHI data from PVGIS

Summary

It happens that PVGIS provides timeseries with missing data (happening with GHI = 0 during all the day or big art of the day). This problem has not been detected ?yet? for T° and wind speed.

Steps to reproduce

Latitude: 50.376, Longitude: 5.677, LocationName: Siguesol, Year identified as problematic: 2008 (23/01/2008, GHI = 0 all day and the two previous days also)

What is the current bug behavior?

The diffuse irradiation computation crashes when a day has missing data (GHI is 0 all day).

What is the expected correct behavior?

Should we just accept that a day has no irradiance at all ?

Or should we aim at completing the data with some average or something ? (e.g. averaging window of a few days before and after)

The data-filling strategy will also depend on the amount of missing data. Should we prepare for more than a day of missing data ?

Relevant logs and/or screenshots

See #190 (closed)

Possible fixes

• Use daily irradiance data (in kJ/(m²·d)) from Agri4cast to fill the data gaps in PVGIS for whole-day gaps
• For smaller gaps (1 hour - few hours) : interpolation ?
• switch weather database (e.g. ERA5)

To do

• Contact PVGIS team at JRC to have their opinion/advice on missing data (waiting for reply)
• in the code :
  • check for missing days (is there at least 1 day with no irradiance ?)
  • fix it (tbd)
• edit the wiki appropriately