A Steppable wrapper which can be stopped. This is a convenience class for those situations where you'd
like to schedule an agent to get stepped once in the future, but then think better of it
and would like to prevent it from happening. Wrap your agent in the TentativeStep, then
schedule the TentativeStep on the Schedule. When you want to prevent the agent's step() method from
being called, simply call stop() on the TentativeStep. When stop() is called on a TentativeStep, it
sets its underlying agent to null and forgets about it. Note that the TentativeStep itself is still
scheduled, and so at some point the Schedule's time step will advance to that point even if the
underlying Steppable has been removed from the TentativeStep.
double scheduleTime = ...
Steppable mySteppable = ...
TentativeStep tent = new TentativeStep(mySteppable);
Now, to stop mySteppable from being called before scheduleTime has come, you'd say: