export-world should export a full model system

[arthurhjorth]

Calling export-world iteratively down the tree of child models is not difficult, but we will need to figure out some way of keeping track of, and dealing with exceptions relating to LevelSpace not being able to find the right .nlogo files. @qiemem do you have any suggestions?

[qiemem]

Hmm, I had kind of dismissed the possibility of export-world ever working, but you're right, traversing the tree is not hard, especially given how flexible exporting extension objects is.

Keeping track of and dealing with missing models shouldn't be hard. I think the hardest part will be just getting the models to serialize properly, with string escaping and the like. I'll look into it.