Remove outputformat parameter in get_pvgis_hourly and get_pvgis_tmy

[AdamRJensen]

Supporting the four different outputformat options in get_pvgis_hourly and get_pvgis_tmy is proving to be a rather high maintenance burden without much benefit. I therefore suggest that we altogether get rid of the parameter and simply choose the json option as this is the easiest to parse and features more or similar metadata as other options. This issue is already mentioned in #2344 and #2399.

The issue in #2344 demonstrates how the json option is the easiest to maintain.

Here's the metadata available when outputformat='csv'
Header:

Footer:

Here's the metadata available when ``outputformat='json```

[kandersolar]

I think outputformat='epw' is worth considering. It returns two additional data columns: 'aerosol_optical_depth' and 'temp_dew'. It's also even easier to parse than the JSON, as it needs only a call to the parse_epw function.

Another question: if we get rid of outputformat in get_pvgis_tmy, what will happen with read_pvgis_tmy?

[AdamRJensen]

As long as we get rid of outputformat='basic' in #2416 I'm ok keeping the others for now.