Optionally return the number of times each event occurred

[jmason42]

Technically all we need after a stochastic simulation run (step or repeated) is time and counts, but often it's interesting to look at which events occurred, too. As an option we should return a vector of the number of each event that occurred, which would either be a series of one-hot row vector (one for each simulated step), or a cumulative sum of all events that have occurred over the simulated interval.

[prismofeverything]

We need this feature to implement a direct translation of complexation in the WCM also, so bumping the priority on this one.

[jmason42]

Largely addressed by #23