FlatGeobuf 3d support

[kylebarron]

Change list

• Use geo-traits implementation from Implement geo-traits on Flatgeobuf geometry objects #779
• Make a couple fixes in the geo-traits.
  • Offsets were coordinate offsets and need to be multiplied by 2 to get the correct x and y values
  • Lengths are end - start
  • I was computing the number of linestrings in multi line strings incorrectly.
• Add more tests on the Python side.

[kylebarron]

This isn't working yet, I think it's because I'm going through geozero, and it's unclear whether the issue is on my side or geozero's side. It may be better to first update to use geo-traits for parsing geometries: #818

[kylebarron]

Now the geo-traits are piped through, but there's something not working on the read side. It looks like it's working for points. So if I had to guess, it's probably that the geo-traits are incorrectly implemented for polygons.

We should add tests with linestring and polygon data to see which geometry types pass.