out-of-core rasterization and zonal stats #129

In rasterix I added some dask-aware wrappers for rasterio.features.geometry_mask, rasterio.features.rasterize and exact_extract (for coverage). I did this for some clients who wanted zonal stats over very large arrays.

I think it makes sense for xvec to use those wrappers under the hood, and simply add a GeometryIndex to the result.

I didn't fully wrap all of exactextract's reductions yet, but this could be done, ideally in combo with flox.

Warning: this work is lightly tested.