Introduction of CARTAGO Signal Events

Signals are events that are generated by Cartago artifacts. They will be modelled in ASTRA as custom events @cartago(signal, formula f) where f is a predicate representing the signal.

e.g. for the Counter artifact, there is a tick signal - this would take the form:

@cartago(signal, tick())

to make this work, i will need to refactor CartagoPropertyEvent to be CartagoEvent and to add the signal type...