Added support for special nodes that only send when data is ready

[ostrosco]

• A new macro: create_aggregate_node! constructs nodes that must output Option types. Data is only sent from the transformation function if the data is not None. If the data is None, the node just moves to the next iteration.
• Fixed errors in the testing framework where the asserts weren't actually running since the main thread would terminate and not wait for the children thread to finish.
• Added a test case for aggregate nodes and a very rudimentary test case to get some estimates of message throughput through the nodes to see if crossbeam will work for our particular use case.