Add MFD flow length computation (#1009)

New flow_length_mfd() computes proportion-weighted flow path lengths
from MFD fraction grids. Downstream mode gives the expected distance
to outlet weighted by MFD fractions; upstream mode gives the longest
path from any divide to each cell.

All four backends: NumPy, CuPy, Dask+NumPy, Dask+CuPy.
29 tests covering correctness, edge cases, and cross-backend consistency.

Author: brendancol

xrspatial/__init__.py

@@ -45,6 +45,7 @@
 from xrspatial.flow_direction_dinf import flow_direction_dinf  # noqa
 from xrspatial.flow_direction_mfd import flow_direction_mfd  # noqa
 from xrspatial.flow_length import flow_length  # noqa
+from xrspatial.flow_length_mfd import flow_length_mfd  # noqa
 from xrspatial.flow_path import flow_path  # noqa
 from xrspatial.focal import mean  # noqa
 from xrspatial.glcm import glcm_texture  # noqa