Only return final state by default when running until a time

[jmason42]

While histories are generally interesting, as part of a larger simulation suite we generally neither need nor want the full history of events. Thus a run-until stochastic simulation need only return the final counts, and, to keep with our pattern, the final event time (which would naturally be no later and typically earlier than the run-until time). Storing and returning the full history would need to be specifically enabled by a user.