Consider exactextract as backend #109
Description
Consider exactextract as backend for zonal statistics. Seems like their list of statistics is comprehensive, and the library should be performant (written in C++).
As a bonus, they are compatible with rasterio and xarray out of the box. The biggest downside is a lack of documentation, but that may not be a big deal.
Ultimately, we should go with what makes most sense from the benchmarks (maybe even allow both backend types?)