Migration from signed stock-flow to CLD

[tim-at-topos]

To-do:

• write some tests
• make those tests pass

[tim-at-topos]

@epatters This test fails, but for "bad reasons" — the models are indeed actually the same, but they are ordered differently. Would you prefer me to

1. Change the test to simply iterate through all objects and all morphisms and compare them one by one, removing any matches from each model as we go and ensuring that we end up with two empty models?
2. Actually implement some general method for checking models are equal "up to reordering of HashIndex"?
3. Something else entirely?

I suppose that one thing that (3) could be is building a visualisation analysis that displays the CLD, but this is roughly as non-trivial as (2) imho (for somebody like me), so I'd rather save that specific suggestion for a future PR