Rules to deal with state (transition) failures

[norro]

Implement rules to deal with state (transition) failures, i.e.

• a (sub-)system or node is not in its intended state/mode
• transition of a (sub-)system or node fails

[norro]

This will probably go into the mode-manager as well. We just had the idea that behavior trees might be a good way to model these rules.

[norro]

Merged to dev, marked as experimental in the documentation.