QOL improvements for DisjointLists

[wbernoudy]

Now Model.disjoint_lists() returns only the DisjointLists symbol. The DisjointList successors can be accessed by indexing/iterating the DisjointLists symbol.

This is backwards-compatibility breaking.

[arcondello]

I agree we should implement DisjointLists.__getitem__().

For the return value, I agree this is much better but unfortunately changing the return value is a significant backwards compatibility break so we'd want to save that for a major release.