Add contour line extraction via marching squares (#964) (#974)

* 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.

* Fix marching squares case table and add tests (#964)

Corrected edge assignments in the marching squares lookup table.
Previous table had wrong edges for most cases (e.g. case 1 used
top edge instead of bottom edge). Added comprehensive test suite
covering correctness, NaN handling, edge cases, backend equivalence,
GeoDataFrame output, accessor integration, and closed-ring detection.

* Add contours to docs and README feature matrix (#964)

* Add contour extraction user guide notebook (#964)

* Remove unused _MS_TABLE and _interp_edge from contour module (#964)

* Rename contour notebook to 24_Contour_Lines (#964)

* Add interactive contour explorer standalone example (#964)

Matplotlib-based interactive app for exploring contour lines on
procedural terrain. Supports click-to-add levels, scroll density
control, filled contours, hillshade toggle, contour labels, index
contours, and colour scheme cycling.

Author: brendancol

README.md

@@ -257,6 +257,7 @@ 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/polygonize.py) | Converts contiguous regions of equal value into vector polygons | ✅️ | ✅️ | ✅️ | 🔄 |
+| [Contours](xrspatial/contour.py) | Extracts elevation contour lines (isolines) from a raster surface | ✅️ | ✅️ | 🔄 | 🔄 |
 
 --------
 

docs/source/reference/utilities.rst

@@ -25,6 +25,13 @@ Polygonize
 
     xrspatial.polygonize.polygonize
 
+Contours
+========
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.contour.contours
+
 Diagnostics
 ===========
 .. autosummary::