We would like to add a step() method to simulators.cpp, that will let a
Python script advance the simulator one report step at a time. Before
calling step(), the Python script will have to call a step_init()
method that initializes the simulator, similar to what is done in run()
now. In order to avoid duplication of code in run() and step_init() the
run() method is here refactored into a prepareRun_() function that can
be called from both step_init() and from run().
