pvlib.iotools.get_pvgis_hourly outputs are not ModelChain.run_model_from_poa friendly

[ramaroesilva]

Is your feature request related to a problem? Please describe.
Even with map_variables=True, the get_pvgis_hourly retrieves the POA diffuse split by sky and ground components (while ModelChain.run_model_from_poa expects only poa_diffuse), which asks for users to sum the components manually beforehand.

Describe the solution you'd like
Would be nice to have something more streamlined. Maybe when poa_sky_diffuse and poa_ground_diffuse are available, have the summation happen within the ModelChain?

Additional context
This might relate/be impacted by #1553.

[cwhanse]

Current behavior would fail with ValueError if data is missing poa_diffuse. I don't think it's a good idea to quietly accommodate data with missing columns, even when it seems obvious how to fill in the missing columns for a user. If ModelChain was promising to handle inputs directly from weather data sources such as PVGis, I'd feel differently about it.