Time events are not handled in model exchange example (listed on the standard doc)

[clagms]

I provide a PR that shows this problem with a modified bounding ball model (called BouncingBallStairs): #70

I also fixed this issue in #71

The expected behavior of the BouncingBallStairs model is as follows:

Essentially, each time there's a bounce, the model schedules a time event for about 0.1s later.

The model also shows that, when an timed event happens at the same time as a state event, it can happen that the state event is not longer handled. That is, if the ball collides with the floor (the importer detects a zero crossing), but the floor moves, then the ball will no longer collide with the floor.

The behavior obtained from #70 is the incorrect one: