selection.data() should return all the data, not just the first group’s

[mbostock]

It’s kind of weird either way, but it feels like selection.data only returning the first group’s data makes selection.data (with no arguments) only useful in the case of a flat selection. So we might as well make it return all the data, flattened into a single array. That would make selection.nodes more useful, and symmetric, as well.