update a node, and its associated links, vertical positions (y0, y1)
function adjustNodeHeight (node, dy, sankeyY0, sankeyY1) {
    if ((node.y0 + dy >= sankeyY0) && (node.y1 + dy <= sankeyY1)) {
      node.y0 = node.y0 + dy
      node.y1 = node.y1 + dy

      node.targetLinks.forEach(function (l) {
        l.y1 = l.y1 + dy
      })

      node.sourceLinks.forEach(function (l) {
        l.y0 = l.y0 + dy
      })
    }
    return node
  }