Add dask, cupy, and dask+cupy backends for polygonize, update README

Polygonize now supports all four backends. The cupy and dask+cupy
backends use a hybrid GPU/CPU approach: GPU arrays are transferred to
CPU for boundary tracing, which is inherently sequential. The dask
backend chunks independently, merging cross-boundary polygons.

Author: brendancol

README.md

@@ -212,7 +212,7 @@ In the GIS world, rasters are used for representing continuous phenomena (e.g. e
 
 | Name | Description | NumPy xr.DataArray | Dask xr.DataArray | CuPy GPU xr.DataArray | Dask GPU xr.DataArray |
 |:-----|:------------|:------------------:|:-----------------:|:---------------------:|:---------------------:|
-| [Polygonize](xrspatial/experimental/polygonize.py) | Converts contiguous regions of equal value into vector polygons | ✅️ | | | |
+| [Polygonize](xrspatial/experimental/polygonize.py) | Converts contiguous regions of equal value into vector polygons | ✅️ | ✅️ | ✅️ | ✅️ |
 
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