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.

Author: brendancol