Deprecate `flow_accumulation` and `drainagebasins`

[wkearn]

These can both be accomplished efficiently using flow algebras (currently in src/streamquad.c:

• flow_accumulation is traverse_down_{f32, f64}_add_mul
• drainagebasins is propagatevaluesupstream_i64 or traverse_up_u32_or_and

This removes some internal duplication and reduces the surface area of the libtopotoolbox API.

These functions will need to be removed from their uses in pytopotoolbox, topotoolbox3 and topotoolboxr before being removed from libtopotoolbox.

See #164, #211, TopoToolbox/pytopotoolbox#367 for more discussion about this change and #210 (review) for more discussion about this design.