Add a simple co-simulation example with GridKit apps operating as ZMQ client/server.

Summary

Create a simple example where a 3-bus system operates as a ZMQ client and constant signal source operates as ZMQ server.

CC @alexander-novo @lukelowry

Rationale

This example is a prototype for implementing capability for GridKit apps to operate in a co-simulation environment.

Description

Use 3-bus example in GridKit and modify it to replace one of the generators with a signal adapter. Implement that app as a ZMQ client expecting signal input from a generator implemented as a ZMQ server.

In this initial implementation, use constant signal source as a generator model. The signal source should be implemented as a separate app, a ZMQ server, and should return (constant) values for injection currents on client's request.

Additional information

Another example, working through DataBroker instead of direct connection between the two GridKit apps, should be added, as well.