GridAgent should handle termination of ParticipantAgent

[sebastian-peter]

The GridAgent creates all ParticipantAgents that are connect to the corresponding subgrid. Once created, the GA also watches the corresponding actors, meaning that it receives a Terminated message. This message is currently not handled, meaning GridAgent fails in an unexpected way.

With akka/pekko typed, the syntax will change a little bit, but the principle stands (receiveSignal).