Exit cleanly when ODE step size near zero

[mnneely]

Simulator is crashing when ODE step size near zero. This can happen if sampling from given mean/SD results in negative parameters. While truncated or log-transformed simulations can avoid this, if neither option is chosen by user, this error can easily arise if CV% is large.

Simulator should exit cleanly and provide informative message with possible solution.

[mhovd]

Thanks Michael - do you have any additional information on when the simulator crashes?
E.g. which model with which dosing with which parameters?

I have been working on making the simulator fail gracefully, but trying to balance performance as well.