Add an Agent to the Agents container.
an exception is thrown if the AgentIdentifier has already been added before to the Agents
No value should be sent back to the sender from within this method
Agents understand the following messages: virtufin.simulation.Agents.ListAgentIdentifiers, virtufin.simulation.Agents.ProcessMessages, virtufin.simulation.Agents.AddAgent and virtufin.simulation.Agents.RemoveAgent