Standardize pvgis_hourly to return (data,meta)

[AdamRJensen]

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The pvlib.iotools.get_pvgis_hourly and read_pvigs_hourly currently does not follow the iotools standard convention returning a tuple with (data,meta). Instead the functions currently output (data,inputs,meta), which is causing a lot of confusion for new users, particular this is a major pain point during intro tutorials. This PR introduces impelements a breaking change of by changing the output of the functions to follow the iotools standard convention.

[kandersolar]

This change (move the inputs dict to be within the returned metadata dict instead of a separate variable) is okay with me.

A few minor suggestions below. Otherwise, my only feedback is that it would be nice if the descriptions of the metadata dict in Returns were more useful.