Add spatialpandas interface and polygon rasterization

[philippjfr]

• Add spatialpandas interface
• Add canonical conversion for all geometry data formats
• Add support for rasterizing geometries
• Add docs
• Add tests

[philippjfr]

Handling geometries consistently turned out to be a pretty hard problem taking me most of last week and most of today. Some of the features here:

• .iloc will now work consistently throughout (meaning that Selection1D will work nicely)
• .groupby is now handled consistently throughout
• .sort is now implemented allowing us to sort by any dimension other than a geometry dimension
• .values can now be used consistently to access both the geometry and value arrays (previously you had to split to access them)
• Polygon and Ring geometries now always return closed rings from .values
• All geometry interfaces can now be converted between one another meaning that they will even roundtrip nicely
• Behavior for the native dictionary format is now well defined for different geometry types
• Shared testing infrastructure for all geometry interfaces including MultiInterface (and all its subtypes), SpatialPandasInterface, GeoPandasInterface (in geoviews)

[philippjfr]

Some of the Path plotting code looks pretty terrible now but it should all become much cleaner once I pick up #3986 again.

[philippjfr]

This is ready to review.

[philippjfr]

Thanks for the initial review @jlstevens, I've now made sure that only actual geometry interfaces are supported by Path types and reverted the API change on .split.

[jlstevens]

@philippjfr Other than some comments above (most of which you have addressed already), I am happy to merge this PR now.

[jlstevens]

Looks good. Thanks!

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