Fix render transitions

[akosel]

Fixes regression discussed in https://github.com/dagrejs/dagre-d3/issues/321.

The commit to update to d3.js v4 brought in a change that forces a complete redraw of the graph (due to removing the DOM tree underneath the graph element. Additionally, a number of other locations were mixing up entering v exiting v updating, so I cleaned that up a bit. Also, fixed an issue in the exit function for edgePaths that causes the createLine call to always fail as it passes in an empty object, which will always lead to an undefined curve function being called.

[lutzroeder]

Thank you for looking into this.

Tried these steps with new painting over existing:

1. Build and open interactive-demo.html
2. Replace definition with digraph { X [ label="Foo" ]; }
3. Changed the label to Bar

[akosel]

Thank you for looking into this.

Tried these steps with new painting over existing:

1. Build and open interactive-demo.html
2. Replace definition with digraph { X [ label="Foo" ]; }
3. Changed the label to Bar

This should now be fixed.

[lutzroeder]

The same seems to be happening with edge labels, changing the font or adding characters.

Example 1

Example 2

[akosel]

Ah, good catches. Ok, that second one is now fixed. If you see anything else please keep sending them along. I'm less familiar with all the edge cases, so this is helpful.

[akosel]

Ah wait, just noticed the second image there seems to be a separate issue. I'll add something for that too.

[akosel]

Ok, both of those cases are now handled.