Replace `flow_accumulation` with `traverse_down_f32_add_mul`

[wkearn]

See TopoToolbox/libtopotoolbox#212

This will need a new C binding, but it is otherwise quite similar to the flow_accumulation API. The only difference, I think, is that you initialize the acc array with the weights rather than supplying the weights separately.

I can take care of this, @ktlenz.