Sketch of reading from GeoArrow struct encoding

[kylebarron]

This is a sketch (only point and multipoint are implemented for now) of reading from the proposed struct encoding in the geoarrow spec.

GeoArrow is undecided as of now whether the point object should be represented by a logical "struct" (physically, the x and y arrays are distinct in memory) or a logical "fixed size list" (physically, x and y coordinates are interleaved). This PR implements the struct approach; it's not hard to switch between the two, so we could implement both for now and then remove one if/when the geoarrow spec chooses one for the standard.

Notes:

• If feedback is good, I'd work to add the rest of the types. It's pretty straightforward, though ensuring offsets are all correct can be a little tedious (and will need tests ofc)
• Arrow extension types are used to clarify the meaning of various data types. For example a List<Struct<x: f64, y: f64>> (i.e. a logical type representing a variable size list of x,y structs) could either represent a multipoint or a linestring, so we need some metadata to describe which it is. Arrow2 has an example here of using extension types. Geoarrow extension type names are defined here.
• Not sure how to represent null geometries through geozero. In arrow, every value has a null bitmap, so every geometry can be null
• In the long term, I still want to work towards geo traits, but this seems a worthwhile stepping stone.

Related to https://github.com/georust/geozero/issues/37

[pka]

Code looks fine for me so far!

geozero has very limited support for empty (null) geometries. See this discussion for more details: https://github.com/georust/geozero/pull/20#discussion_r840003700

[kylebarron]

For now I switched to prototyping a geoarrow implementation as a crate in geopolars so that I could iterate faster, and in the future I think it would make sense to come back and figure out how to integrate with geozero.

I suppose I could also just implement GeomProcessor in the geoarrow crate and keep it as a separate implementation, like the flatgeobuf crate is.

[kylebarron]

Closing this because this was implemented in https://github.com/geoarrow/geoarrow-rs, with bindings from that into geozero