[d3-sankey] Option to explicitly assign nodes to stacks

[thiloplanz]

The layout() function automatically assigns nodes to stacks ("computeNodeBreadths") according to their distance from the sources.

It would be nice to have some option on the node to explicitly override this to manually arrange nodes into stacks. (Behaviour when that is inconsistent with the direction of flows can be undefined).

[smartinsightsfromdata]

I support this.
To me, without overcomplicating the issue, it would be nice in case to just add a column with the position / level desired for the representation on the X axis (from left to right).

Enzo