Floating point exception when calling model.step

[FabianSchubert]

float point exception (core dumped) is returned when calling model.step_time() if the model was loaded with num_recording_timesteps=0 instead of num_recording_timesteps=None. This is not a major issue, as one can just use None in this case, but it should probably not present an error to the user. Rather, one would expect the same behaviour if either None or 0 was used.