Create a `WorkerGraph` class to enable "worker programming"

[mathemaphysics]

As discussed in #13 we would like to also create a WorkerGraph class which allows for more than simply linear processes to be constructed. The end result is a kind of topological program flow. The ideal is that for each Worker there will be a running in the ProcessMessage function which is capable of discovering something about the message data and sending the resulting message to any number of other processing accepting that type of message.

Because of the unidirectional flow of messages, the resulting graph will be a directed graph, although it is obviously possible for messages to flow from one Worker's output to another Worker's input and vice versa.