Update to vega datasets v2

[eitanlees]

This is a big update! I think I covered all of my bases.

• Add football.json.
• Add penguins.json.
• Add seattle-weather-hourly-normals.csv.
• Update weather.csv and seattle-weather.csv with better encoded weather condition, indicating more rain. Thanks to @visnup!
• Update co2-concentration data and add seasonally adjusted CO2 field.
• Switch to ISO 8601 dates in seattle-weather.csv.
• Rename weball26.json to political-contributions.json.
• Convert birdstrikes.json to birdstrikes.csv and use ISO 8601 dates.
• Convert movies.json to use column names with spaces use ISO 8601 dates.
• Remove climate.json.
• Replace seattle-temps.csv with more general seattle-weather-hourly-normals.csv.
• Remove sf-temps.csv.
• Remove graticule.json. Use graticule generator instead.
• Remove points.json.
• Remove iris.json. Use penguins.json instead.

I think this will call for a major version bump as well, but I don't know much about how python packaging works 😛

[eitanlees]

I will work on fixing up this PR and let you know when it's ready again to be merged.

[eitanlees]

Alright, all tests are passing and I think everything is good to go! @jakevdp