Add contour line extraction via marching squares (#964)

New xrspatial.contour module with contours() function that extracts
isolines from 2D rasters. Supports numpy, cupy, dask+numpy, and
dask+cupy backends. Includes segment stitching into polylines and
optional GeoDataFrame output.

Author: brendancol

xrspatial/__init__.py

@@ -1,5 +1,6 @@
 from xrspatial.aspect import aspect  # noqa
 from xrspatial.balanced_allocation import balanced_allocation  # noqa
+from xrspatial.contour import contours  # noqa
 from xrspatial.bump import bump  # noqa
 from xrspatial.corridor import least_cost_corridor  # noqa
 from xrspatial.cost_distance import cost_distance  # noqa

xrspatial/accessor.py

@@ -313,6 +313,10 @@ def polygonize(self, **kwargs):
         from .polygonize import polygonize
         return polygonize(self._obj, **kwargs)
 
+    def contours(self, **kwargs):
+        from .contour import contours
+        return contours(self._obj, **kwargs)
+
     # ---- Fire ----
 
     def dnbr(self, post_nbr_agg, **kwargs):