Fixes #733: add cost_distance() for weighted proximity (#859)

* Fixes #733: add cost_distance() for weighted proximity via multi-source Dijkstra

New cost_distance() function computes minimum accumulated traversal cost
through a friction surface to reach the nearest target pixel, the raster
equivalent of GRASS r.cost / ArcGIS Cost Distance. Supports numpy and
dask+numpy backends, 4/8-connectivity, max_cost truncation for dask
scalability, and xr.Dataset input via @supports_dataset. Includes 19
tests and an example notebook comparing cost_distance with proximity.

* Add cost_distance to README feature matrix

* Add k-d tree dask path for unbounded proximity to avoid single-chunk rechunk

The existing dask proximity path rechunked the entire raster into one chunk
when max_distance was unbounded, defeating dask's out-of-core purpose. This
adds a two-phase scipy.spatial.cKDTree approach: Phase 1 streams chunks to
collect target coordinates, Phase 2 queries the tree per-chunk via map_blocks.
Exact results, memory proportional to targets not raster size. Supports
EUCLIDEAN and MANHATTAN metrics; GREAT_CIRCLE/ALLOCATION/DIRECTION fall back
to the existing single-chunk path. Gracefully degrades when scipy is absent.

Author: brendancol

README.md

@@ -192,6 +192,7 @@ 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 |
 |:----------:|:------------|:----------------------:|:--------------------:|:-------------------:|:------:|
 | [Allocation](xrspatial/proximity.py) | Assigns each cell to the identity of the nearest source feature | ✅️ | ✅ | | |
+| [Cost Distance](xrspatial/cost_distance.py) | Computes minimum accumulated cost to the nearest source through a friction surface | ✅️ | ✅ | | |
 | [Direction](xrspatial/proximity.py) | Computes the direction from each cell to the nearest source feature | ✅️ | ✅ | | |
 | [Proximity](xrspatial/proximity.py) | Computes the distance from each cell to the nearest source feature | ✅️ | ✅ | | |