Revert "use a generator in nearest_on_boundary"

This reverts commit cea097c2b3d4a71a141d07ca4b4cd7a8750527ad.
reimplement GeoJSONReader

in a way that maps closer to the GeoJSON standard

rather than having `pull_[geom]` methods, the new class has a single unified
`items` method
cast null to NaN in vector properties
Merge branch 'geojson-refactor' of github.com:njwilson23/karta into geojson-refactor

Conflicts:
	karta/vector/read.py