Fix for large padding

[antoinerg]

As shown in this Codepen using Plotly.js which is wrapping d3-sankey (https://codepen.io/anon/pen/vvgYEx) when nodePadding is too large, nodes end up with negative heights. This PR fixes this by reducing the padding appropriately to use at most 2/3 of the total vertical space.

[mbostock]

Hey, thanks for the suggestion and sorry for the delayed response. This PR includes the built files (which shouldn’t be checked in), and the implementation also mutates the value of py, whereas what we should be doing instead is using a different value for py while computing the layout, such that the specified sankey.nodePadding is preserved if the layout is called again (and no reduction in the padding is required). I’m going to close this PR and try to tackle this issue myself but if you are still inclined to contribute, please reopen this PR or submit a new one. Thank you!