Add container property to ElkEdge

[tuckergordon]

ELK#775 added a container property to edges:

Write a container property into each edge after layout. This gives the id of the node that was this edge's container, and helps the user interpret the route points in the right coord system.

Updating the ElkEdge type to reflect this and avoid type errors.

Note to reviewers: not sure if this type belongs in ElkEdge, or ElkExtendedEdge. Based on the upstream PR, I think this should be a non-optional property but happy to make changes!

[soerendomroes]

Thank you for this PR. We are in the process of improving the container property. I am however currently ill and so do not feel alarmed if it takes a while.

[tuckergordon]

Thank you, I appreciate that and all the work you've put into this library! I hope you feel better!

[wirekang]

container should be an optional type. As far as I understand, it is written during layout() process, so "input" edges don't need to have container property.

[skieffer]

it is written during layout() process, so "input" edges don't need to have container property.

That's correct. Furthermore, since eclipse-elk/elk#1071 (which supersedes the PR referenced above), the container property is written into edges only when CONTAINER coordinates mode is used. If ROOT or PARENT coordinates are used, then the property is not written at all, even after layout.

[skieffer]

I think the functioning of the container property is now settled enough that this could be merged.