selection.data key with simple function

[daenenk]

To make the API useful

 svg.selectAll[Node](".node").data(force.nodes(), (d: Node, i: Int) => d.id)

should compile, but (d: Node, i: Int) => d.id is not lifted or recognised as a js.thisFunction2

I've read http://stackoverflow.com/questions/27577368/d3-js-key-function-running-twice-on-simple-selector-array-combo. The current definition might be technically spoken correct but of little use if you don't overload it as to accept the above. Instantiating from js.thisFunction2 may be useful to mimic this binding from js in scala. But to pass a closure as argument, it just makes things complex. The point made on stackoverflow is that the passed function should work both on the selected data as well on the new data.
In case of

 svg.selectAll[Node](".node").data[N1,N2](force.nodes(): js.Array[N1] , (d: N2, i: Int) => d.id)

The type relation should be: N2 >: N1 and N2 >: Node
or when N2 == Node then N1 <: Node

as such I would propose both, next to the thisFunction based API if you like

def data[NewDatum <: Datum](data: js.Array[NewDatum], key: js.Function1[Datum, String]): Update[NewDatum] = js.native
def data[NewDatum <: Datum](data: js.Array[NewDatum], key: js.Function2[Datum, Int, String]): Update[NewDatum] = js.native