Implement proper state event detection in OMSimulator

[casella]

Currently, OMSimulator only checks for event-triggering conditions (e.g. zero-crossing functions changing their sign) at each time step, but doesn't try to figure out the exact time when the zero-crossing function changed sign. This results in delays of up to one time step in the triggering of events.

In many cases this is no big deal, but in some cases this may seriously change or disrupt the simulation results.

We should eventually implement a proper state event detection mechanism in OMSimulator.