add edge to model like 'action-possible-conditional'

[Morgan Bentell]

There should be an edge type that describes a relation between data and action that makes an action possible/allowed if a given data is present, and a predicate function is true.

An example where this would be useful is for example in a model of a legal process. The two actions 'release' and 'serve-punishment' would never both be relevant. Only one of them would be applicable depending on the result of a predicate function that takes the data 'verdict'

I think this is an important feature that the library has been missing that really allows you to model logic gateways.