Problems with selection.data/selection.datum of d3-selection

[shamansir]

I treated selection.datum the wrong way, I though that it just sets/gets the __data__ property and that's it. But by d3 philosophy is actually overwrites __data__ for a child when it is appended, see:

https://github.com/d3/d3-selection/blob/e3b5455107/src/selection/append.js#L5
https://github.com/d3/d3-selection/blob/e3b5455107/src/selection/select.js#L9

Also see the issue and the comment: d3/d3-selection#106 (comment)

It affects the way I build HTML: for example, I create an HTML/SVG element corresponding to the Patch, set some data to it, and then at some point I create an HTML/SVG element corresponding to the Node, set another data to it, and append it to the parent Patch element. It turned out, at this point of appending, d3 literally overwrites the __data__ property of a child with a data from a parent (see code above).

At some point, I'll try to follow d3 philosophy (#441), but for now I will add my own __rpd_data__ property and some function to access it easily and to replace it with something right in the future.

[shamansir]

Required for #440 / #405

[shamansir]

Fixed with #440