Add Dinf support to flow_accumulation() (#945)

flow_accumulation() only understood D8 integer codes. Passing Dinf
angles from flow_direction_dinf() silently produced wrong results
(every cell got 1.0) because float angles never matched any D8 code
in _code_to_offset.

Auto-detect D8 vs Dinf from input values, then run Tarboton's
proportional splitting for Dinf. All four backends covered: numpy,
cupy, dask+numpy, dask+cupy. 45 tests pass (24 existing D8 unchanged,
15 new Dinf, 6 detection).

Author: brendancol