Sometimes simulator::set_xxx() is called while simulator::do_step() is still in progress

[kyllingstad]

In fixed_step_algorithm::impl::do_step() we loop over all simulators and call their individual do_step() functions, storing the resulting future<step_result> objects in a container. We then loop over all simulators again, call get() on the aforementioned futures, and transfer their outgoing values.

However, in the process of transferring, we not only call the current simulator's get_xxx() functions, we also call other simulators' set_xxx() functions. We can't do that, because we may not have gotten around to calling future<step_result>.get() for some of those simulators yet, and then we don't know that their do_step() has completed.

The solution is to get all the step_results before transferring variables. Also, some asserts should be sprinkled around slave_simulator to prevent this from going unnoticed in the future.