Show convergence bottleneck nodes to users

[SouthEndMusic]

Fixes #452.

To do:

• Implement Show convergence bottleneck nodes to users #452 (comment),
• Add some more basins to the model and test that the quickly emptying one is the bottleneck.

[SouthEndMusic]

I've implemented this now, but the result is not what I expect. The problematic basins (which get drained extremely quickly) in the test model have error $0$, and the others have some positive error. We need to think about under which circumstances, say which values of retcode this method makes sense.

Edit: I get the expected basins now.

[evetion]

LGTM