Proposal: Allow a topology to contain other topologies.

Note that this may be recursive.

Use case 1: Describe a large network with geographically separate regions (such as Nordunet)

Use case 2: This makes it easier to describe an abstracted topology to peers

Use case 3: This may remove the need to introduce virtual nodes (artf6487)

Use case 4: describe (the dataplane of) a federation of multiple domains. This may provide a means for scaling in path 
finding.

Use case 1 can also be solved with geo-information, but is likely more scalable. Use case 2 can also be solved on the 
application level.